By: Amy Grant
If you have an avocado tree that is so rife with fruit, the limbs are in danger of breaking. This may lead you to wonder, “Should I thin my avocado fruit?” Avocado fruit thinning is similar to thinning other fruiting trees, such as apples. So how do you thin avocado fruit? Read on to learn more.
Columnar cultivars of avocado are pinched at an early age to attain a more rounded habit, but most other types of avocado require no training and little pruning. Any pruning of avocado that may be done is done so judiciously since avocado trees are susceptible to sunscald, which results in defoliation. Avocado fruit is also self-thinning, so thinning avocado fruit is generally not required.
While thinning isn’t normally required, several cultivars of avocado are in the habit of bearing fruit in alternate years. That is, in a particular year, the tree produces a staggering amount of fruit, so much that the energy from the tree either cannot support the enormous quantity or the resulting yield is high but fruit is small. In the following year, the tree’s energy is so depleted that it barely fruits, if at all.
In this case, it may be advisable to lightly thin the fruit. Also, thinning is advisable when multiple trees begin to grow together such that their canopies begin to lose light.
When trees are bearing overly heavily, they often drop a lot of fruit before it reaches maturity and any fruit that is left behind is often of a small size. Removing some avocado fruit will allow the tree to expend energy on the remaining avocadoes, resulting in larger fruit.
Avocado fruit is borne in clusters, sometimes just a few and sometimes many fruit are growing together. Take a good look at the grouping of immature fruit and identify any that are misshapen, diseased or pest damaged, and the smallest fruit. These are the fruit you will remove, leaving just the largest, healthiest looking avocado in the cluster.
Using sharp bypass pruners, snip off the immature fruit at the stem. I know it’s hard, but continue in this way until you have evenly spaced fruit on the tree. Space fruit about 6 inches (15 cm.) apart on the tree. If you have a cluster of fruit very close to the one that has just been thinned, it is best to remove it rather than thinning to one fruit.
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This article was co-authored by Andrew Carberry, MPH. Andrew Carberry has been working in food systems since 2008. He has a Masters in Public Health Nutrition and Public Health Planning and Administration from the University of Tennessee-Knoxville.
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Avocados — the smooth, creamy, nutrient-filled fruit that is essential to dishes like guacamole, can be grown from the pit that is leftover after eating the fruit. Though avocado trees grown from a pit can take quite some time to produce fruit of their own (sometimes as long as 7-15 years), growing an avocado tree is a fun, rewarding project that leaves you with a great-looking tree in the meantime. Once your tree is grown, you can wait for avocados to begin growing or jump start the process by grafting or budding productive plant limbs to your tree. Regardless of the method you choose, learn how to grow your own avocados from scratch by starting with Step 1 below!
By: Pam Elam
University of California Cooperative Extension Farm Advisor
It is often tempting, after eating a particularly good orange or avocado, to plant the seed and grow our own tree full of these delicious fruit. Trees grown from these seed, however, may produce fruit that are not edible at all, or the trees may not bear fruit for many years. The best way to produce good-quality fruit is to grow seedlings from them and then attach, by budding or grafting, material from trees that are known to be good producers. Budding and grafting can also be used to change or add varieties to mature citrus or avocado trees, a process known as top working.
This publication is a brief introduction to budding and grafting for the home gardener. For more information, consult the materials listed at the end of this publication or contact your local Cooperative Extension office.
The best time of year to start citrus or avocado seedlings is in early spring. To germinate citrus or avocado seed, plant them in a shallow container such as a nursery flat or a pan with drainage holes in well-drained commercial potting mix. Plant the seed two to three times deeper than their length. For example, a citrus seed about ¼ inch (6 mm) long should be planted about &half to ¾ inch (12 to 18 mm) deep. Keep the seed in a warm place-between 70° and 80°F (21° to 27°C)-and keep the soil moist. Covering the nursery flats with clear glass or plastic will help maintain the proper humidity. Avocado seed can also be germinated by suspending them in water. Place toothpicks horizontally into the seed near the top. Suspend the wide end of the seed in a small container of water with the toothpicks resting on the edge of the container. Place it in indirect light and refresh the water at least weekly.
After germination (usually 12 to 15 days), replant the seedlings into a larger container of good-quality commercial potting mix. (If all danger of frost has passed, the seedlings may be planted directly into the ground where you want the tree to grow instead of replanted into containers.) Good choices for containers include a cardboard milk carton cut horizontally in half or a one-gallon can. Punch drain holes in the bottom of the container. The seedling will be ready for budding or grafting when it has grown to 24 to 30 inches (60 to 75 cm) tall.
Keys to Budding and Grafting
Budding and grafting are vegetative propagation techniques in which a single bud or stem (scion) of a desired plant (cultivar) is attached to a rootstock plant. In budding, a single bud with its accompanying bark (often referred to as budwood) is used as the scion. In grafting, part of a stem or branch is used as the scion. One of the most important keys to successful budding and grafting is properly positioning the scion on the rootstock. In order for the scion and rootstock to grow together, the thin greenish plant layer (cambium) just under the bark of the scion and rootstock must be aligned so that they touch each other. If they do not touch each other, the bud or graft will fail. Within 10 to 15 days, a successful bud or graft forms a hard whitish tissue (callus) where the two cambium layers grow together.
Always use sharp cutting or grafting instruments and make clean, even cuts. Options include a budding knife, a sharp kitchen knife, or a single-sided razor blade. Do not allow the cut surfaces of the scion or rootstock to dry out. Immerse cut scions in a pail of water, wrap them in plastic, or graft them immediately after cutting. Also, remove any leaves from scions after cutting to help keep the scions from losing water. Keep the scions in a cool place during the work.
When to Bud or Graft
Budding and grafting are best done in the spring or fall when the bark is easily separated from the wood. It should be timed to be early enough so that warm weather will help ensure a good bud union, yet late enough so that the bud will not begin to grow and callus will not grow over the bud itself. Citrus budded or grafted in the fall must be protected from frost. Avocados are best grafted in the spring when the bark is easily separated from the wood.
Budding is the standard method used to propagate citrus. Aside from being the easiest method, it allows a large number of plants to be propagated from a small amount of scion wood and is suitable for trees, rootstocks, or branches from 1 /4 to 1 inch (0.6 to 2.5 cm) in diameter.
Budwood should be taken only from high-producing, disease-free trees (see Warning at end of this article). The best citrus budwood is located just below the most recent flush of new growth the best avocado budwood is located near the terminal end of shoots that have fully matured, leathery leaves.
How to make a T-bud
T-budding (see fig. 1) is generally the best budding method for citrus and avocados. To make a T-bud, make a T-shaped cut on the rootstock about 8 to 12 inches (20 to 30 cm) above the ground (fig. 1A). The vertical part of the T should be about 1 inch (2.5 cm) long and the horizontal part
about one-third of the distance around the rootstock. Twist the knife gently to open flaps of bark. Avoid cutting through any buds on the bark of the rootstock.
On the scion (fig. 1B), cut a selected bud beginning about 1 /2 inch (1.2 cm) below the bud and ending about 3 /4 to 1 inch (1.9 to 2.5 cm) beyond the bud. Make a horizontal cut about 3 /4 inch (1.9 cm) above the bud down through the bark and into the wood. Gently remove the shield-shaped piece for budding (fig. 1C).
Slip the budwood down into the T-shaped cut under the two flaps of bark until the horizontal cuts of the bud match up with the horizontal cut of the T (fig. 1D). After inserting the budwood into the rootstock, wrap the bud and rootstock with budding rubber (fig. 1E). Budding rubber is available from agricultural supply or hardware stores if budding rubber is unavailable, use wide rubber bands, green tie tape, or stretchy tape. Leave the bud exposed while wrapping. Do not coat the area with grafting wax or sealant.
If the budding is done in the fall, the buds should be healed in about 6 to 8 weeks in the spring, healing should take about 3 to 4 weeks. After the bud has healed, unwrap it and cut off the remaining shoots or stock about 12 to 14 inches (30 to 35 cm) above the bud union. This will be the nurse branch, which helps protect the new bud union. After the budwood has grown a few new leaves, completely remove the nurse branch to about 1 /8 inch (3 mm) above the bud union (fig. 2).
The best grafting technique for small-diameter 1/4 to 1 /2 inch [0.6 to 1.2 cm]) rootstocks is whip grafting. Whip grafting should be done in the fall or spring. Although whip grafts use more scion wood than budding does, they allow the grafted plant to develop more rapidly.
To make a whip graft (fig. 3), select as a scion hard and mature green wood. First make a long, sloping cut about 1 to 2&half inches (2.5 to 6.2 cm) long on the rootstock (fig. 3A). Make a matching cut on the scion. Cut a "tongue" on both the scion and rootstock by slicing downward into the wood (figs. 3B-3C). The tongues should allow the scion and rootstock to lock together. Fit the scion to the rootstock (fig. 3D) and secure with budding rubber (fig. 3E). Apply grafting wax to seal the union. To prevent sunburn, new whip grafts should be protected from the sun until they heal. After the scion has begun to grow, remove any growth from the rootstock. If necessary, support new shoots by staking.
The best grafting technique for large-diameter trees or branches is bark grafting (fig. 4). To make a bark graft, first cut off the rootstock (the trunk or branch to be grafted) just above a crotch where smaller branches sprout out. If possible, try to retain one branch of the original plant as a nurse branch. The nurse branch will provide the scion nutrition and support from wind (the nurse branch will eventually be removed).
Cut vertical slits 21/2 to 3 1/2 inches (6.2 to 8.7 cm) long through the bark of the remaining freshly cut rootstock stubs down to the wood. These slits should be spaced 3 to 5 inches (7.5 to 12.5 cm) apart. Cut the scions 5 to 6 inches (12.5 to 15 cm) long with 4 to 6 buds per scion (figs. 4A-4C). If scions are cut longer than this, they may dry out before healing. When cutting the scions, make a sloping cut about 3 inches (7.5 cm) long at the base of the scion.
Using a grafting knife or other very sharp knife, lift the bark on one side of the slit. Insert the scion into the slit with the long-cut surface of the scion facing the wood of the rootstock and push it down into the slit (fig. 4D). Make sure that the scion fits snugly into the slits in the bark and that the cambiums are properly aligned.
Secure citrus scions by nailing them in place with thin flathead nails or tying them with strong cord or tree tape. Secure avocado scions with plastic nursery tape. Coat all cut surfaces thoroughly, including the tops of the scions, with grafting wax or pruning paint. To protect the graft from sunburn, paint it with white interior water-based paint, either undiluted or mixed 50/50 with water. Paint the entire area around the graft union, including the scions, waxed areas, and the exposed trunk below the graft union. Inspect the grafts frequently and re-wax them if they begin to crack or dry out.
Once the scions begin to grow well, remove all but one scion per branch. Early on, however, prune the scions that will be removed to reduce their vigor but do not prune the scion that will be kept. The one scion you keep will eventually become a main scaffold branch. Any nurse branches should also be removed after all the scions are growing well.
Top working is the process of changing fruit varieties on a mature tree. Most citrus and avocado are top worked by bark grafting (see above). Top working should be done in the spring or fall.
WARNING: It is against California law to bring citrus budwood into the San Joaquin Valley from any area outside the valley. This law protects commercial citrus groves from disease caused by the Tristezavirus. Budwood or scion wood should be collected only from citrus that has been tested in the last year and found to be free of Tristezavirus. Consult your local county Cooperative Extension office for more details..
Garner, R. J., and S. A. Chaudri. 1976. The propagation of tropical fruit trees. East Malling, Kent, England: Commonwealth Bureau of Horticulture and Plantation Crops, Horticultural Review No. 4.
Hartmann, H. T., and J. A. Beutel. 1994. Propagation of temperate-zone fruit plants. Oakland: University of California Division of Agriculture and Natural Resources Publication 21103.
Hartmann, H. T., and D. E. Kester. 1975, Plant propagation: Principles and practices. 3rd ed. Engelwood Cliffs, N.J.: Prentice-Hall.
Reuther, Walter. 1973. The citrus industry. Vol. 3. Oakland: University of California Division of Agriculture Sciences.
Whitsell, R. H., G. E. Martin, B. O. Bergh, A. V. Lypps, and W. H. Brokan. 1989. Propagating avocados: Principles and techniques of nursery and field grafting. Oakland: University of California Division of Agriculture and Natural Resources Publication 21461.
Tasaki, Ernia. 1985. All about citrus and subtropical fruits. San Ramon, CA: Ortho Books
My attitude toward protecting avocado trees during winter cold snaps has evolved. I once lived close to the beach where I never thought about protecting my avocados in winter. Then I moved inland and watched the temperatures dive at night but shrugged at first, “Well, if the tree can’t survive, it doesn’t belong here. I’m not going to pamper it.”
I now realize the shortcomings of that philosophy. In Southern California, you just need to get a young avocado tree through its first couple winters and then it can withstand all but the rarest and coldest arctic blasts we receive. And adding a couple degrees of warmth on a couple nights each winter is usually all it takes.
This post is about when avocado trees need cold protection, what protective measures I’ve tried in past winters, and new methods I will test this winter, if necessary.
At what temperatures do you need to protect avocado trees?
I have a thermometer on my porch where I estimate that the temperature stays around 4 degrees warmer than out in the open parts of my yard. Over the past handful of winters, I’ve recorded some of the low temperatures on that thermometer along with damage caused to avocado trees.
Damage to young and small avocado trees, roughly younger than three years in the ground and under five feet tall, or unhealthy avocado trees can happen once the temperature drops below freezing, 32 degrees. But I’ve never seen damage to mature and healthy avocado trees until the temperature drops below 30. This is the threshold for observed avocado damage in my yard.
On the day after Christmas in 2015, the yard got down to 29 and I noticed a couple of newly emerged leaves on various avocado trees partially burned. There was frost on the ground throughout the yard that morning.
In late February of this year, 2018, the yard got down to 27 and new leaves on avocado trees were damaged, plus some older leaves showed brown mottling, and most flowers on all of my avocado trees were killed (except some deep inside the canopy). The frosty photo at the top of the page was taken during this event.
During the last days of 2014 and the first days of 2015, the yard got down to 26. I refer to this event as the New Year’s Freeze of 2015, and it remains the lowest temperature reached in my yard during the five winters I’ve spent here. (See video update from winter 2020 at bottom.) Damage to my avocados was widespread although not serious except to young trees.
There was not only cold burning of avocado leaves on all varieties, including Fuerte and Sir-Prize, but also branch tips killed back up to six inches in a couple parts of the canopy of my Reed avocado tree. As far as young trees went, a Pinkerton that had been in the ground for less than a year had all of its leaves and all but one of its limbs killed back to the trunk. (I hadn’t protected the Pinkerton in any way.)
The Pinkerton, after suffering many hours in the mid to upper 20s.
(This was an unusually cold event, when snow fell in parts of southern avocado country: Escondido, Valley Center, and Temecula. See photos of that snow in avocado groves at this blog post by Reuben Hofshi.)
–Consider protecting young and small or unhealthy avocado trees when it’s predicted to drop to about 32 degrees
–Consider protecting mature and healthy avocado trees from about 30 degrees
–Don’t anticipate significant damage to mature and healthy avocado trees (fruit and limb damage) until below 27
Preparing an avocado tree for imminent cold: Irrigate
If there’s a weather alert that temperatures will drop below freezing, step one is to be sure your avocado tree is anchored in soil that is moist. Water as necessary. There are two reasons for this: One, an avocado tree in moist soil will be hydrated and best able to handle the stress of low temperatures. Two, moist soil is able to store more heat from the sun during the day, which means it can also radiate that heat at night. (See this interesting article from the University of California, “Passive Frost Protection of Trees and Vines.” )
Related to the soil’s ability to store heat and radiate it later, I’ve read advice to remove mulch from beneath the tree since mulch blocks soil from warming as much as possible. This might make sense. I’ve observed the pattern innumerable times in my yard and neighborhood where, during light frosts, mulched (with wood chips or compost) areas are covered by frost while adjacent bare soil is not.
Even so, last winter I experimented with removing mulch from below some of my trees and found unclear results. During the frosty nights of late February 2018, some mulched trees were damaged, some were not. Some unmulched trees were damaged, some were not. I don’t know what conclusions to draw from this, honestly.
Methods of protecting avocado trees from frost and freeze: Overhead coverage
Now for what to do to protect your avocado tree during a frigid night. A small tree is easily protected by putting a roof over its head. Any overhead covering is warmer than nothing.
I’ve used blankets and rugs.
I’ve also used nylon tarps on top of ladders set over the tree, trash cans, cardboard boxes, and more. I’ve seen that they all keep the tree at least a few degrees warmer, in part as judged by the frost forming on ground near such protected trees but not under them, in addition to their sustaining no damage.
How does overhead protection work, and how to use it? The way this method works is by trapping radiation that emanates from both the tree and the soil below the tree. At night, the tree and soil are releasing a certain amount of heat into the air, and if you can block that radiation you can keep the temperature around the tree higher than it otherwise would be.
Higher by how many degrees? I’ve read estimates of 4-8 degrees. It seems to depend on the materials used. Regardless, even if it’s only two degrees higher, a young tree kept at 31 compared to 29 can mean the difference between no damage and all leaves lost.
(See an article from 1951 by Don Gustafson called “Frost Protection of Young Avocado Trees,” showing growers in Orange County using burlap covers.)
Will you trap even more heat by wrapping the sides too? Sure thing. It’s like the difference between sleeping under a mere tarp and sleeping in an enclosed tent.
The above photo shows a young avocado tree wrapped in Reflectix bubble insulation (see it on Amazon here). I happen to have some of this material on hand so I’ve used it to protect trees in winters past.
The one thing to keep in mind about using both overhead protection and wrapping the tree entirely is that they should be put in place before sunset to be most effective. You want to begin trapping the radiation before it gets too cold. Also, you want to remove them in the morning once sun is on the tree, unless it’s a structure like my wood board above, which lets light onto the tree and soil. That structure could actually be left in place all winter. (Great option for frost protection if you’re going on vacation.)
Another material that is deliberately made for insulation, but this time specifically for insulating plants, is a frost blanket such as this one.
N-Sulate frost protection blanket.
I bought this N-Sulate frost blanket at a nursery for $13. It’s also available online, such as on Amazon here.
Frost blankets can entirely wrap a small tree, and if they do, blankets of this weight (1.5 ounces per square yard) claim to be able to give 6-8 degrees of protection. But they can also give somewhat less protection if used to partially cover a larger tree. I’ve got a good crop of fruit on my Lamb avocado tree that I don’t want to lose, so if temperatures threaten to damage this fruit I’ll throw the blanket over it like this.
I secured the frost blanket to small branches using clothespins.
View inside the frost blanket.
This frost blanket allows a lot of light to enter, as you can see in the photo above. Because of this, you could safely leave it on a tree during the day. (Another good option for frost protection if you’re going out of town during winter.)
By the way, if you had a couple of smaller avocado trees that you wanted to protect, you could completely wrap them with a pack of smaller frost blankets, such as these 7’x7′ ones made by Agribon, available through Peaceful Valley Farm Supply (Grow Organic.com).
An additional way to add warmth to a larger tree is the classic Christmas lights approach. Run an extension cord out to the tree, then string Christmas lights (or similar lights) through the tree. Leave the lights on all night, especially through the early morning when it’s usually coldest, to add a few degrees of warming.
Here are some lights that would work for this purpose: Brightown Globe String Lights (here they are on Amazon).
Note that they are incandescent and not LED, which wouldn’t give off the desired heat.
I set them up for a show on this Nimlioh tree.
Of course, you can use lights on small trees as well. In fact, both covering a tree and turning lights on under the covering adds maximum heat. You can also use other sources of heat placed below the tree, such as a shop light, a table lamp, or a space heater.
Some people claim that you can get some protection by leaving buckets of water under or next to the tree. When water freezes it releases a small amount of heat. I tried this one night a long time ago and found no effect compared to nearby trees that spent the night bucketless. I’m not convinced it is effective, especially compared to other techniques.
Taking advantage of this same principle is the idea of running sprinklers under a tree throughout the night. As the water freezes it releases some heat. In order for this to work, the sprinklers must be run continuously all night. I’ve never tried this because other methods seem better suited for a home garden situation. Large groves do make use of this method though. (See this article by Ben Faber of the University of California which briefly notes how to use sprinklers on avocados for frost protection.)
–Any overhead coverage raises the temperature around the tree a few degrees.
–Overhead plus side coverage keeps a tree even warmer.
–Lights strung in a tree add warmth.
–Lights or other heat within a covering would add maximum heat and protection of an avocado tree in the cold.
When a winter night gets cruel and it’s below 40 already at dinner time and protecting your avocado tree is called for, is it worth the effort? Yes. I’ve lost young trees in the past that I might be eating from now if only I’d thrown a little protection over them on one or two nights.
Prepare now. Have the materials on hand. This advice is directed at myself as much as anyone, for I’m especially exposed to the cold this winter as I’ve got more small young avocado trees to shepherd than usual. Can we please get a warm and rainy winter like 2016/2017? Wish me luck, I wish you luck.
Update March 19, 2019:
The winter certainly was rainy but it was not warm. The coldest night in my yard was February 19 when the thermometer dropped to 27. Fortunately, I didn’t lose even the tiniest of avocado trees.
Mostly, I protected them with frost blankets (Agribon AG-50, as seen here). A few trees I helped with only umbrellas, and for a couple young trees I added heat with lights.
Update February 2020:
A night of 25 degrees caused a spectrum of damage to different trees:
Here is a video of the same trees showing their recovery and the effects of the cold after one year:
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There are over 900 named varieties of avocado, how do you pick one? Well for one the selection at commercial retail nurseries is somewhat limited, compared to what is offered at nurseries that growers use. But you do have options. In general, avocado trees are big and need room. ‘Hass’ has an umbrella-shaped canopy, but ‘Reed’ is more columnar and needs less room. ‘Holiday’ is a much smaller tree than ‘Hass’ and “Littlecado’ is a somewhat smaller tree. You can also choose on the basis of when you want to pick your fruit. For example, ‘Fuerte’ is a winter fruit and ‘Reed’ is a summer fruit. That means they taste best in those periods, but they will hold on the tree for a much longer period of time. You can also choose on the basis of taste. Of course, this is very subjective, but ‘Zutano’, ‘Bacon’ and ‘Stewart’ have a lower oil content than ‘Hass’, ‘Pinkerton’ and ‘Reed’. Many consider ‘Nabal’ the best tasting fruit. It’s a summer variety and hard to find in retail nurseries, but sometimes you can order them for delivery later.
You can read more about varieties at the website: http://ucavo.ucr.edu/AvocadoVarieties/VarietyFrame.html#Anchor-47857 . But remember, most of these are not available at retail nurseries.
How to tell when an avocado is ripe
The avocado is an amazing fruit. It grows on a tree and comes to maturity, reaches certain oil content and a stage at which it will ripen, but does not ripen on the tree. It needs to be removed from the tree before it will soften. If the fruit is removed before it has reached maturity it will not soften, and will remain rubbery and inedible. One of the problems is that the fruit will hang on the tree for an extended period of time and it is hard to know when they are mature. Avocados are not like apricots where you have about 2 weeks to get the fruit off before it falls off. As the fruit stays on the tree in gradually develops more and more oil content and has a richer flavor.
If the fruit stays on the tree too long, the oil can develop and almost rancid flavor, however. So it is good to know when the best, acceptable flavor is. Avocado varieties fall into general seasonal periods when they are mature –‘Fuerte’ in winter, ‘Hass’ in spring/summer, ‘Lamb-Hass’ in summer/fall. Or you can pick a fruit and put it on the counter and watch to see if it softens evenly. If it does in a two week period, the rest on the tree are good to go.
I just raked up all the leaves under the avocado and it looks so nice. PUT THEM RIGHT BACK. The avocado is shallow-rooted and really depends on the natural leaf mulch to protect its roots. In fact the roots will actually colonize the rotted leaves as if it were soil. This mulch is also a first line of defense against root rot. The decomposing leaves create a hostile environment to the microorganism that causes the disease. The mulch also helps to reduce evaporative loss of water and therefore reduces water needs. Avocado growers will actually spread mulch in cases where trees are too young to produce adequate leaf drop for mulch or in windy areas where mulch has blown away. The key to remember is that the mulch should be kept at least six inches away from the trunk to avoid collar rot which can be causes by keeping a moist mulch against the trunk.
Mature avocados may be a big tree, but they have very shallow roots. The bulk of them are in the top 8 inches of soil. The tree therefore does not have access to a large volume of stored water. As opposed to a deep rooted walnut, they need frequent, small amounts of water. A young tree in the summer might need multiple applications per week, but because the root system is small, each application may only be 5-20 gallons. An older tree with its wider rooting pattern may go a week to a month between irrigations depending on the weather and rainfall. Proper irrigation is the best way to keep the avocado from getting root rot. Both over and under irrigation can induce the conditions for root rot, although over irrigation is more common. And remember, it is not just the amount applied at an irrigation, but the timing that is important, as well. Because you are managing such a shallow root system, just poking your finger into the root system will tell you if there is adequate moisture there before you irrigate again.
The canopy is thinning. The leaves are small and yellow. There is dieback in the canopy, with leafless tips on the branches. You dig around under the canopy in the wetted area of the sprinkler and you can’t find roots within 6 inches of the soil surface or if you do find them they are black. There is little mulch under the tree. There are weeds growing under the tree. All these are signs of root rot disease. But it is also a sign of lack of water, because that is what is happening – there are no roots to take up water. And one of the things a gardener will often do, is start watering the diseased tree more, thinking it is lack of water, which if it is diseased only makes the condition worse. Adding more water to a tree that can not easily take it up only creates an asphyxiation that makes matters worse. Irrigation and mulch are the two most important factors for avoiding the disease.
So what do you do if you have disease? There are fungicides that are available from the nursery, but there are a number of things that you can do before applying something like that. First of all get a handle on the irrigation. Make sure you are irrigating to the tree’s needs. Check soil moisture before irrigating. Make sure the tree is not getting supplemental water from another area such as a lawn sprinkler. Make sure there is a good thick, woody mulch under the canopy. Adding gypsum (15-20 pounds per tree) evenly spread under the canopy can also help, but reviewing and modifying the irrigation practice is the most important thing that you can do.
The most important thing you can do before planting is assess the pH of the soil before planting. Avocados are very sensitive to soil pH greater than 7. Their uptake of iron and zinc can be terribly compromised and they will suffer. Correcting the soil pH prior to planting is the easiest way to approach the problem, rather than trying to correct it later when the tree is in the ground. Then it is expensive and takes a long time to correct the problem without killing the tree. Elemental sulfur (not popcorn sulfur) in pellets is the easiest way to accomplish this. Watering and waiting for the sulfur to make the change and then checking to make sure the pH is really down takes about 6 months.
As for nutrients, though, the most commonly required ones are nitrogen and potassium. These can be applied as either an organic (fish, soybean, manure, etc.) or synthetic forms (urea, ammonium sulfate, potassium sulfate, etc.) or a combination. The thing to remember is that the tree because of its shallow root system likes small, frequent amounts of nutrients. And because it is a subtropical plant, it goes quiescent in the winter and when the soil is cold. So nutrient applications like nitrogen, are best applied when soils are warm spring though fall.
Newly planted trees do not need supplemental nutrients, they are loaded up from the nursery and the young root system can easily burn until it gets established. The first year use the equivalent of 1 ounce per tree total of nitrogen in 4- 5 applications over the spring/fall. The following year double that amount and do so for each succeeding year for the next 5 years. The more small applications you make the less total nitrogen fertilizer you will use. Use the equivalent amount of nitrogen whether it is a synthetic source or an organic source. Once the tree has started to develop a thick leaf mulch, it is possible to back off on nitrogen applications because now the mulch is contributing some of the nutrients. In many garden situations where mulch is maintained, by year 10 the nutrient status is self-sustaining and nitrogen fertilizer may not be needed at all. Just keep an eye on the leaf color to make sure it stays green, indicating adequate nitrogen.
When the avocado starts bearing fruit about year three, it may need potassium. This is not necessary in all situations throughout the state, but the harvested avocado fruit contains twice as much potassium as nitrogen and when the fruit is removed the tree can start showing potassium deficiency symptoms. This can be analyzed for at a lab, but probably the best thing to do is just apply potassium sulfate at an equivalent rate to nitrogen or to use triple 15 fertilizer to meet both the nitrogen and potassium needs. For organic growers can use organic potassium sulfate or kelp. Although phosphorus is used by avocados, there are no documented cases in California where supplemental phosphorus needs to be applied.
The avocado is an unusual beast in many ways. And flowering is no exception. It follows what is called synchronous dichogamy. The flower has both male and female parts, but those portions open at different times, opening first as female, closing and then opening as a male. It does this over two days, so in effect it can not pollinate itself. To make it more interesting there are what are called A and B varieties. These varieties open and close in a different pattern, so that there is some overlap between the male stage of one variety and the female stage of another variety. This is how you get cross pollination. He sounds really good as a model, but the avocados didn’t read the book, since for a given variety there are always some laggards and there are often both female and male stages on the same tree.
Currently in the Florida area there is a pest-disease complex called Laurel Wilt Disease. This is caused by a fungus that is carried by an ambrosia beetle. The beetle is extremely small and carries the fungus in its mouth parts. The beetle bores into the wood, spreading the fungus. Most of the beetles are females. They lay their eggs in the wood, the fungus starts growing, the eggs hatch and the larvae feed on the fungus growing on the wood. The problem with this fungus is that grows, clogging the vessels that carry water (xylem) and the tree dies from lack of water. This happens very rapidly, in under a year. At this point, there are really no effective sprays to control the beetle or fungicides to control the fungus. This pest-disease complex is thought to have come into Savannah harbor on untreated wood packing crates from somewhere in Asia. If it gets to California, it will be very difficult to control.
Avocados in general are very frost sensitive, however backyards tend to be warmer than open fields and homeowners can often grow trees with less concern than growers. All young trees are more susceptible to freeze damage than a tree that has a full canopy down to the ground. The canopy traps heat and keeps the tree warmer than the surrounding air.
When trees are young they are small enough that frost cloth can be framed around the young tree. This usually provides at least 5 degrees of protection. Just a blanket can be used, as well as long as a structure is built to keep it off the tree. When trees get older and are too large to cover, homeowners have used a sting of lights in the canopy to generate heat. Make sure they are lights designed for the outdoors.
There are differences in how much cold a tree will take. ‘Hass’ and ‘Holiday’ are very sensitive, while ‘Stewart’ and ‘Zutano’ are much more cold tolerant. Here is a comparative chart of the different varieties