Aster Seed Sowing – How And When To Plant Aster Seeds


By: Ilana Goldowitz Jimenez, Plant Scientist & Writer

Asters are classic flowers that typically bloom in late summer and fall. You can find potted aster plants at many garden stores, but growing asters from seed is easy and less expensive. So why not get some seeds and add fall color to your garden?

Aster Seed Growing

Asters are a group of perennial flowers that belong to the Asteraceae family, also called the daisy family. Many species and varieties, both wild and cultivated, are available to gardeners. This means you have a lot of options, including tall or short plants with flowers in many different shades of blue, purple, pink, or white.

In North America, asters provide valuable food sources for butterflies, native bees, and other insects. They are a great choice for wildflower and butterfly gardens and for planting in meadow habitats.

Most asters prefer cool, moist climates, especially at night. Many need a cool or cold winter to grow back the following year. For example, the New England aster is very cold hardy and grows best in zones 3-8.

When to Plant Aster Seeds

The best time for outdoor aster seed sowing is just after the last frost in your area. You can also start seeds indoors using a good seed starting mix four to six weeks before the last frost. Indoor aster seed care involves keeping the seeds at a temperature of 65-70 degrees F. (18-21 degrees C.) and providing seedlings with ample light as soon as they emerge.

How to Grow Aster Flowers from Seed

First, choose an appropriate planting site. Asters will do best in full sun, but many varieties can also grow in partial shade. Well-drained soil is best.

Prepare the planting site, especially if it’s a new garden bed, by mixing in compost, composted manure, or another source of organic matter and nutrients.

If you’ll be planting outdoors, follow the seed spacing instructions for your variety. Many asters can be spaced 3 inches (8 cm.) apart, then thinned to 12 inches (30 cm.) apart after they emerge.

Whether planting indoors or outdoors, cover the seeds with 1/8 inch (0.3 cm.) of fine soil. Aster seed sowing by scattering the seeds in a wildflower planting is also perfectly fine. Water the seeds after planting, then keep them evenly moist until seedlings emerge. This can happen between 7 and 21 days after sowing, depending on the aster variety.

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Sapphire Mist aster. (Symphyotrichum dumosus).
Photo by Proven Winners.

Zones:

3-10 varies depending on species and cultivar.

Flower color:

Colors are diverse, ranging from white and pastel blues and pinks, to hybrids of deep scarlet and purple.

Bloom time:

August through October varies depending on species and cultivar.

Height/spread:

Most range from 1 to 4 feet in height, with some growing as tall as 5 or 6 feet and some staying as small as 6 inches. Their width typically ranges from 1 to 3 feet.

Types:

According to the Chicago Botanic Garden, there are approximately 250 types, many of which are native to North America.

Here is a list of the most common garden asters:

  • New England (Symphyotrichum novae-angliae)
  • New York (Symphyotrichum novi-belgii)
  • Aromatic (Symphyotrichum oblongifolium)
  • Heath (Symphyotrichum ericoides)
  • Smooth (Symphyotrichum leave)
  • Calico (Symphyotrichum lateriflorum)
  • Wood (Eurybia divaricata)

Other, less common, types include alpine, California, tatarian, heart-leaved, big leaf, sea, short’s, crooked-stem, Frikart’s, Italian, and East Indies. China aster (Callistephus chinensis), which belongs to the same plant family discussed here, is actually an annual.


How to Care for an Aster

Care for your asters with some basic rules of thumb, and be rewarded with spectacular blossoms in a variety of colors. Planting these wonderful stars (aster is Greek for star) will uplift and brighten your fall garden. This Michaelmas daisy (another name for the flower) produces blue, white, red, purple, pink and lavender flowers. Ranging from 8 inches to almost 8 feet, these flowers make good border plants, but be careful of mildew diseases that attack them.

Propagating Asters

Asters can be grown indoors or may be sown into the garden directly (providing danger of frost is well past). Plant them in early spring, preparing gardens with a tiller to loosen soil. Sow the seeds approximately 1 foot deep into a mixture of compost and garden soil. Germination usually occurs after approximately 1 month.

Plant in well-drained, moist soil in either partial shade or morning sun. Some varieties can be planted in full sun, but this varies, so be sure to check planting instructions. Compost, peat moss or mulch will retain moisture, control weeds and ensure plants have sufficient nutrients.

Your hole for transplanting asters should be twice as wide and deep as the plant's container. Plant the crown of the aster even with the ground level. Plant and thin plants to at least 18 inches apart to avoid overcrowding

Caring for Asters

Divide mature plants in spring, just as the new shoots begin to grow. This should be done every few years to avoid crowding of plants.

As with many other flowering plants, dead head (cut back spent flowers) to make room for newer blossoms. This will extend the health and flowering of your plants. Be sure to dead head early on in the blooming season. Blooming will be reduced if done too late. This will also restrain unwanted reseeding which will cause plant overcrowding.

TIP: Our expert gardening advisor, Karen Thurber adds, "To produce a fuller, shorter plant, that requires less staking, pinch your asters when they reach 10 to 12 inches. The first pinch should remove 5 to 6 inches and leave 3to 5 internodes. Asters can be pinched 2to 3 times during the growing season, with your last pinch before July 25th. A pinch later than July 25th will delay flowering."

Remember, asters are prone to mildew so be careful to plant them in areas with good circulation and good sun exposure.

As with other flowering plants, do not allow water to saturate leaves. Be sure to water at the plant's roots to prevent mildew and mold. Drip irrigation and utilization of a soaker hose work very well in watering these plants.

Strategies

In addition to accenting rock gardens, grow your asters in succession, so that have an ongoing blooming season. Planting asters will also deter some insects in your garden. Plant them throughout your garden to limit pesticide use.

Asters attract butterflies, which will enhance the beauty of your garden.

Unlike marigolds and similar plants, aster seedlings are unlike the parent plant and may not be desirable to allow to reproduce. Remove the flower stems before they set seed to prevent this cycle from occurring.

Water asters as directed, but remember that they are considered a "drought tolerant" plant and do not like standing water.


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