How to Grow Aster in Pots: The Fresh Soil Rule and Winter Blooms

China aster is the long stemmed winter cut flower in pink, purple and white that fills bazaar bouquets, and you can grow it in a pot if you respect one rule above all: use fresh soil every year. Sow in October or November on the plains, transplant at 4 leaves into a 5 litre pot of new mix, pinch once, and you can cut flowers roughly from February into March.

Why do asters suddenly wilt and die?

This is the question that sends most aster growers to search, so it goes first. A healthy looking plant droops one hot afternoon, doesn’t recover by evening, and the stem base turns brown. Soil borne wilt and stem rot are the usual culprits, and they live on in the soil after the plant is gone. NC State’s extension toolbox lists wilt and stem rot as the problems to watch and gives the plain advice: grow China asters in a different place each year, and destroy diseased plants.

In a pot that means: never reuse last season’s aster soil for asters. Tip it into a bed where you’ll grow something unrelated, or sun it under clear plastic through May and June, and start the asters in fresh mix.

How do I grow aster from seed?

1. Tray: fill a seed tray with a fine, fresh mix (half cocopeat, half sieved compost). Our seed starting guide covers trays and light.
2. Sow thinly and cover with about 3 to 5 mm of mix. Water from below.
3. Germination usually takes 7 to 14 days in October warmth.
4. Transplant at 4 true leaves into pots of fresh mix: 2 parts soil, 1 part compost, 1 part sand or cocopeat. Plant at the same depth, not deeper.
5. Pinch the tip once when the plant has 6 to 8 leaves to get more stems; skip pinching if you want one big flower per stem.
6. Support tall varieties with a thin stick by January.

Seed or nursery seedlings for a beginner?

Nursery seedlings are a fair shortcut for asters, because the risky part is the tiny seedling stage. The catch is that you can’t see the nursery’s soil hygiene. Buy sturdy, green seedlings without dark, pinched stems at the base, and pot them into your own fresh mix. If you have a seed tray and some patience, seed gives you a wider choice of colours and is cheaper.

Sowing and bloom by zone

How much water and feed?

Asters like steady moisture but hate wet stems. Water the soil when the top 2 cm is dry, around every 3 to 4 days in December and every 2 days by March, always in the morning. Don’t let the pot sit in water. On frost nights move pots under shelter; young plants are tender. A spoon of compost on top in December, then a balanced liquid feed every 3 weeks once buds show, is enough. Too much nitrogen gives floppy stems.

Why is my aster not flowering?

• Late sowing: plants sown in December on the plains often hit heat before buds open.
• Short days and fog: January fog slows them; they catch up in February.
• Too much shade: under 5 hours gives tall, weak, flowerless plants.
• Root trouble: yellowing from the bottom and a dark stem base means wilt, not hunger.

Other problems: aphids, leafhoppers, odd green flowers

Aphids cluster on buds; rinse them off or rub them away. If flowers come out green, twisted or stunted with yellowed leaves, that pattern matches aster yellows, a phytoplasma disease spread by leafhoppers. University of Wisconsin extension describes it and says infected plants should be removed promptly, because no spray fixes them. Pull the plant and bin it. If you decide on any insecticide, read the label, use gloves, and keep children and pets away until dry.

How do I save aster seed?

Leave two or three of the best flowers to dry on the plant. When the centre turns fluffy and brown like a tiny dandelion, cut it into a paper bag. Rub out the seeds, which are small and slim with a tuft, and store them dry. Seed from hybrid packets may not come true in colour or height, and seed doesn’t carry the wilt, but the soil does.

Grow it or skip it?

Grow it if you want winter cut flowers and are willing to use fresh soil each year. Maybe if you have only one or two pots and no room to rotate. Skip it if you reuse the same pot of soil every season; you’ll be disappointed by March. Pot over ground on a roof, ground if you have a sunny bed that never grew asters. For more winter colour see Phool aur Paudhe and our potting mix ratios. A tougher companion for the same pots is salvia.

Aster growing questions

Sources

• NC State Extension Gardener Plant Toolbox: Callistephus chinensis
• University of Wisconsin Extension: Aster yellows