By: Darcy Larum, Landscape Designer
Both creeping phlox (Phlox stoloniferais, Phlox subulata) and tall garden phlox (Phlox paniculata) are favorites in flower beds. Large patches of pink, white, purple, or blue creeping phlox are a cheery sight in spring when most other plants are just waking up from their winter slumber. Tall phlox can dominate the summer garden with long lasting, continual blooms that draw butterflies, bees, and even hummingbirds to the garden. Unfortunately, both types of phlox can be prone to a variety of diseases and pests that can discourage gardeners from growing the charming plants. In this article, we will discuss reasons for phlox yellowing and drying out.
Phlox plants are specifically prone to fungal diseases such as southern blight, rust, powdery mildew, etc. Powdery mildew is the most common fungal disease of phlox plants. This disease is first noticed by the powdery white spots or coating on plant tissues. The disease may progress to phlox yellowing and drying out, as well as excessive leaf drop.
Fungal diseases can deplete phlox plants of vital nutrients and water by interrupting the plant’s natural flow of xylem and phloem and its ability to photosynthesize properly. This can lead to yellow or chlorotic and dried out phlox plants.
Nutrient deficiencies, lack of water, improper lighting, and chemical drift can also cause yellow, dried out phlox plants.
In addition to fungal diseases and unsatisfactory environmental conditions, phlox plants can fall victim to viral diseases such as mosaic virus, curly top virus, and aster yellows. These diseases can oftentimes present themselves as phlox yellowing and drying out. Many viral diseases are spread by insects like leafhoppers.
Most fungal diseases are soil borne and infect phlox plants when water from rain or manual watering splashes back up from infected soil onto plant tissues. Watering plants with a slow, light trickle of water directly at the root zone can help prevent the spread of many fungal diseases. However, we cannot control rain; therefore, using preventative fungal sprays before symptoms appear can also be beneficial.
It is also important to provide phlox plants with proper air circulation, prevent overcrowding by properly spacing plants and dividing them frequently, and always clean up and discard fallen leaves and other plants infected with garden diseases.
To ensure healthy plants, phlox should be fertilized regularly, either with slow release fertilizer for flowering plants or monthly foliar sprays. Phlox plants also prefer slightly acidic soil and may not perform well in soils that are too alkaline. Creeping phlox and tall garden phlox grow best in full sun; in densely shaded areas phlox plants may yellow and not grow properly.
Preventative insect control may protect phlox plants from viral diseases. However, when a phlox plant is infected with a viral disease, there is usually no cure. Infected plants should be dug up and destroyed.
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There are three main types of phlox disease. Oedema is a root function problem. Parasitic nematodes (roundworms) are an infection that the plant gets from invasion by roundworms, and powdery mildew is exactly what the name implies: a mildew growth on the plant itself that can kill the phlox if left untreated.
Oedema manifests itself in the form of rusty blisters on leaves and petals. Parasitic nematodes (roundworms) cause foliage and stems to become brown and mottled. Powdery mildew creates a dusty white covering on plant leaves, stems and flowers.
Can I trim it back in the summer to try to rejuvenate it?
You can try giving it a pruning. It is as simple as taking an inch or so off the top, just like a haircut.
I would also try treating it with a fungicide and make sure that it is getting enough water too. In many areas, it has been a hot, dry summer and it may just be going dormant due to this but it also may be a fungus attacking the plant.
Many people find their plants are troubled with powdery mildew, a white fungus that bespeckles or coats the leaves, typically in high summer. Although it is never fatal, it can be unsightly, and may lessen flowering and lead to leaf drop. I first want to remind us how lucky we are that this is the biggest problem with phlox, compared to so many serious problems other plant may have, all those beetles, thrips, soots, aphids, weevils and so forth. Mildew tends to be a greater problem in regions with high humidity, but will also affect phlox that are planted in dry soils. I consider that it is pretty much endemic, and if your phlox are prone to it, it will turn up in every garden sooner or later. What can you do?
Well, to take a passive approach, it is perfectly acceptable to grow other plants in front to hide the unsightly foliage. The mildew that affects phlox is specific to phlox, and will not spread to other species, although if the environmental conditions are favoring phlox mildew, you could well see related different mildews turning up on other plants.
Now, some phlox cultivars are nearly impervious, even in rotten conditions, and others get it at the drop of a spore, even in the best condition. Most of the heirloom varieties have proved extremely resistant, having shown their value to generations of gardeners. Choose varieties that have good mildew resistance, of which there are quite a few available (see list below). Be somewhat cautious in accepting the claims about brand-new cultivars – a skeptical eye has its place, as I’ve learned over the years. I try to grow a new cultivar a couple of years before pronouncing judgment.
This appears to be the most important thing in preventing mildew. I have been observing phlox for 34 years now, and have developed some opinions that are contrary to the standard gardening advice. I don’t thin the stalks or bother to avoid nighttime watering – after all, nature ‘waters’ the plants every night with dew. I have found that, for the most part, happy plants simply resist mildew. Stressed plants will usually develop some degree of mildew. What are the proper conditions that make them happy, healthy and mildew-free?
What makes them happy is to be in cool location, preferably in a little shade, and in a humus-rich soil. Did you know that their native situation is woodland, not out on the dry open prairie? Partially shaded locations are fine, and indeed recommended in climates hotter than Vermont (which is nearly everywhere). If planted in full sun, have other plants grow about the bases, to keep the soils shaded. By all means water them when the weather is dry. The plants should be lifted out every five years or so, and reset in newly and heavily enriched soil (compost or aged manure). Phlox detest hot and dry soils. Keeping a phlox in an exposed hot dry soil, with no surrounding vegetation or mulch, virtually guarantees mildew (and spider mites too), but if you move a plant from hateful to lovable conditions, it should bounce back beautifully the next year, and show little or no spots at all.
There may be times or locations where you need to do more. If you must spray, try the following:
Phlox have very few insect pests. The only pest that is commonly seen is spider mite, which congregates on the underside of leaves, causing leaves to curl, dry up and fall off. You can detect this in the early stages, by observing a slight dusty sootiness on the undersides. They appear to be a problem only when the situation is too hot and dry, and so again it is remedied by improving the soil and increasing the watering. For an immediate response, a good blast of water on the leaves is helpful.
Deer are a huge problem, as they evidently love the taste of Phlox (though it is not to my taste!). I say evidently, because at the nursery we do not have the problem. We are at the village’s edge, and separated from the woods by a broad hay field – evidently the deer don’t think it worth the risk, as they have ample foraging in our rural area. Also, we have a dog that runs off any deer or woodchuck that strays into the nursery. So, I do not have much in the way of personal advice for those of you who struggle with deer. I will instead point you in the direction of websites such as Perry’s Perennial Pages. And again, I would be very interested in hearing of solutions, which I intend to share here.
Q: I have phlox plants in my back yard and every year about this time the leaves start turning yellow from the bottom up. The tops of the plants are fine and they eventually bloom. What is causing this and what can I do about it? The variety is called Miss Mary. - Walt Meidinger, Fargo.
A: Tall garden phlox (Phlox paniculata) are beautiful perennials that are commonly susceptible to leaf blight diseases, including powdery mildew, caused by fungi. Some phlox varieties are more susceptible than others and because it's a widespread problem newer varieties have been developed with resistance or tolerance, usually indicated on the plant tag.
To prevent less-resistant varieties from succumbing, plants can be sprayed with an all-purpose fungicide for flowers and vegetables. Fungicides are best applied as preventatives while the leaves are still healthy.
Fungi are often soil-borne. Remove and discard diseased tops in fall. Avoid overhead sprinkling that splashes soil and wets foliage. Water in the morning, rather than evening. Many phlox growers have found that thinning out the numerous stems increases airflow, reduces disease and promotes larger flower heads. Mulching around plants with a layer of shredded bark keeps soil cool and moist, which phlox prefer.
Q: How do I get rid of creeping Charlie? I literally hand-pulled a garbage bag full over the weekend. This is the worst I've seen them in 20 years. Any advice would be appreciated. - Verna Metzger, Perham, Minn.
A: Creeping Charlie must be having a good year, based on the mailbag. It's a low-growing weed in the mint family with rounded leaves and little purple flowers. It spreads quickly and vigorously in lawns and perennial flowerbeds. Persistence is the key, because a one-time kill is almost impossible.
Several universities are recommending herbicides having a high percentage of the active ingredient triclopyr as being the most effective. It's a broadleaf weed-killer that won't harm lawn grass. If used in perennial flowerbeds, it must be carefully spot-applied to avoid contact with desired plants. Check product ingredient labels for triclopyr.
Apply the herbicide when creeping Charlie is blooming in spring or early summer. Then apply again in September. The fall application is very important. In severe cases, some homeowners have opted to kill everything and start over.
There's an internet remedy being circulated that's recommending borax for creeping Charlie control. Researchers have tested it, found it to be non-effective long-term, plus it can damage lawn grass and other plants.
Q: How much and how often should you water strawberries now that they're producing berries, or shouldn't you water? My plants are June-bearing, still full of blossoms, and berries are about an inch in size, but not turning red yet. - Jean
A: The recommended moisture amount for strawberries is one inch of water per week, either from rain or by sprinkling, which can be applied in one or two waterings. Adequate moisture is especially important when strawberry plants are bearing. When June-bearing varieties are finished fruiting, moisture is less crucial but still important for overall plant health. Everbearing and day-neutral varieties benefit from consistent watering throughout the season.