Gadenin日本語Get the appWhat you get from seed, and what you do not
Seed is the cheap way to fill a border with autumn flower, and for species asters it works well: the seedlings look much like the parent and you can raise fifty plants for the price of one from a pot. It is also the only sensible route if you want a large drift, because dividing your way up to twenty clumps takes several years.
What seed does not do is reproduce a selected garden plant. Anything chosen for a particular flower color, a compact habit or mildew resistance is a selection or a hybrid, and its seedlings scatter back across the range of the parents. Expect washy colors, taller and looser plants than the one you collected from, and no inherited disease resistance at all. If you want more of a specific plant, divide it in spring; that is what division is for.
There is a related nuisance to watch for in the border itself. Many asters seed themselves freely around an established clump, and those volunteers are often more vigorous than the plant they came from. Left alone for three or four years they crowd it out, and gardeners report this as a plant that has changed color when in fact it has been replaced by its own offspring. Deadhead to prevent it, and pull anything growing outside the original crown in spring.
Aster seed also has a short shelf life compared with most flower seed. Fresh seed from the previous autumn germinates well; two-year-old seed germinates patchily and three-year-old seed often not at all. Buy or collect fresh, sow generously, and do not save half the packet for the year after next.
| Item | Target | Note |
|---|---|---|
| Sowing depth | An eighth of an inch (3 mm), barely covered | Sow too deep and germination drops sharply |
| Germination temperature | Around 70F (21C) | Works over a wide range; slower below 60F (15C) |
| Days to emerge | Seven to twenty-one days | Slower and patchier in cool compost |
| Compost moisture | Evenly damp, never saturated | Wet compost plus a covered tray means damping off |
| Transplant at | Two true leaves, past the seed leaves | Handle by a leaf, never by the stem |
| Final spacing outdoors | 18-24 inches (45-60 cm) for tall kinds | Close spacing is where the mildew starts |
| Seed viability | One year reliably, two at a push | Sow fresh; old seed germinates patchily |
Sowing under cover in spring
Sowing indoors or in a greenhouse in spring gives you plants big enough to go out after the last frost and, with the tall autumn-flowering kinds, a reasonable chance of some flower in the first season. Start six to eight weeks before your last expected spring frost. Earlier than that and you end up with drawn, pot-bound seedlings waiting on the weather.
Fill a tray or small pots with fresh, fine seed-starting mix, firm it lightly and water from below until the surface is evenly damp. Scatter the seed thinly across the surface and cover it with no more than an eighth of an inch (3 mm) of sieved compost or fine vermiculite. This is the step people get wrong: aster seed is small and buries easily, and a quarter of an inch of compost over it will cut your germination in half.
Keep the tray at around 70F (21C) in good light. Seedlings appear in seven to twenty-one days, sooner and more evenly at that temperature than in a cool room. Once the first ones are up, take any cover off immediately. A propagator lid left on over emerged seedlings holds humidity against the stems and is the standard route to damping off, which flattens a whole tray overnight and cannot be reversed.
Transplant into individual pots as soon as each seedling has two true leaves beyond the seed leaves, lifting by a leaf rather than by the stem, because a crushed stem never recovers. Grow them on somewhere cooler and bright, then harden them off over ten days or so before planting out after the last frost has passed.
- Sow six to eight weeks before the last frost Working back from your last expected spring frost gives plants of the right size to go out. Sowing much earlier produces drawn, root-bound seedlings that sulk for weeks after planting.
- Sow thinly and cover barely Scatter the seed across firmed, evenly damp compost and cover with no more than an eighth of an inch (3 mm) of sieved compost or vermiculite. Depth is the most common cause of poor germination with aster seed.
- Hold it at around 70F (21C) in good light Expect emergence in seven to twenty-one days. Cooler compost still works but germination is slower and much less even, which makes the tray harder to manage.
- Take the cover off the moment seedlings appear Trapped humidity around emerged seedlings causes damping off, which collapses stems at soil level and spreads across a tray in a night. Ventilate, water from below, and do not let the compost sit saturated.
- Transplant at two true leaves Move each seedling into its own small pot, holding it by a seed leaf and never by the stem. Grow on cool and bright so the plants stay stocky rather than stretching towards the light.
- Harden off, then plant out after the last frost Stand the pots outside for progressively longer over about ten days before planting. Set them 18-24 inches (45-60 cm) apart for tall kinds and water them in thoroughly.
Sow at least twice as many seeds as you want plants. Germination is good but not complete, damping off takes a share of most trays, and you will want the freedom to throw away every weak or leggy seedling rather than planting it out because you are short.
Sowing direct, and sowing fresh seed in autumn
Direct sowing works once the soil has warmed to around 60F (15C) and all frost has passed. Rake a fine seedbed, water the drill before you sow rather than after, scatter the seed thinly and cover with the barest sprinkling of fine soil. Keep the surface damp until the seedlings are through, which in a dry spring means watering with a fine rose every day or two; a seedbed that dries out at the wrong moment kills the whole sowing.
The other option is sowing fresh seed in autumn, as soon as it ripens, into pots left outside in a cold frame or against a sheltered wall. Some aster seed germinates better after a period of cold, and autumn sowing gives it that naturally. The seedlings come up in spring, are noticeably tougher than tray-raised ones, and can go straight out. The cost is a winter of not letting the pots dry out or get waterlogged.
Whichever route you use, label the pot or the row with the date and where the seed came from. Aster seedlings all look identical for their first year, and a tray of unlabelled pots becomes worthless the moment you cannot say which is which.
Volunteers in the border are a third route, and mostly an unwanted one. Seedlings that appear around an established clump can be lifted in spring and grown on if you want more plants, but recognize them for what they are: variable, often vigorous, and not the plant they came from. Anything you do not intend to keep comes out, roots and all, before it starts competing with the parent.
Thinning, first-year care and what to record
Thin ruthlessly, both in the tray and in the ground. Crowded aster seedlings stretch, stay thin and never make a decent crown, and crowded young plants in the bed set up exactly the still, humid conditions that give you a gray lower half in year two. Direct-sown rows come down to a plant every eighteen to twenty-four inches (45-60 cm) for tall kinds; keep the stockiest seedlings rather than the tallest, because height at this stage means stretching, not vigor.
Water new plants thoroughly once a week through their first summer even in cool weather, because a young root system has not spread far enough to find its own moisture. A first-year plant that dries out in midsummer is often set back for a second season as well, and it is the first thing in the bed to mildew.
Do not cut a first-year plant back in early summer. That treatment is for established clumps with plenty of stems to spare; a seedling has three or four and needs all of them. Do not divide either. Leave a seed-raised plant undisturbed until its third spring, then start the ordinary three-year division cycle.
Record the sowing date, the source of the seed, the date the first seedlings emerged, roughly what proportion of the sowing came up, and the position and eventual flower color of every plant you keep. Germination rate against sowing date tells you within two seasons how fresh your seed really was, and the color notes matter because seedlings around the clump will look like the parent long before they flower, and only the written record lets you tell later which plants you put there deliberately.
Keep one line per sowing rather than a tidy summary. Date sown, temperature, depth, days to emerge, proportion germinated. That is five short entries and it turns aster seed from something you hope works into something you can predict.