How to Sow Sweet Alyssum: Surface Sowing, Light and Direct Sowing Outdoors

The seed needs light, and that changes everything

Sweet alyssum seed is small, hard and reddish brown, roughly the size of a grain of coarse ground pepper. Unlike most annuals, it will not germinate reliably in the dark. Light falling on the seed coat is part of the trigger, which means the standard habit of raking seed in or covering it with a quarter inch (6 mm) of soil is the single most common reason a packet fails. Bury sweet alyssum and a large share of it simply sits there and rots.

So the method is to scatter the seed on a firmed, already damp surface and press it into contact with a board or the flat of your hand, without covering it at all. Contact matters as much as light: seed lying loose on dry crumbs dries out between waterings and never gets far enough to root. Firm, damp and uncovered is the combination that works, whether you are sowing into a tray indoors or straight onto a bed outside.

The good news is that once the conditions are right this is one of the fastest and most willing seeds in the garden. In soil at 60 to 70F (15 to 21C) you will see a green fuzz in four to seven days, and a plant flowering about six weeks after that. Because it moves so quickly there is far less reason to start it indoors than there is for slower bedding, and direct sowing where you want the plants avoids the transplant check entirely.

Direct sowing outdoors: the simplest route

Sow direct once the soil has warmed to about 55F (13C) and the risk of a hard frost has passed, which in most temperate gardens is around the time you would sow beans. Sweet alyssum tolerates a light frost as a seedling, so there is no need to be as cautious as with tender bedding, but seed sown into cold wet ground germinates unevenly and gets taken by slugs while it dawdles.

Rake the surface to a fine, level tilth, firm it lightly with the back of the rake, and water it before sowing rather than after. Then scatter the seed thinly by taking pinches and rubbing them out from about six inches (15 cm) above the soil, working across the strip in both directions so you do not end up with a dense stripe down one side. Press the seed down with a board or your palm. Do not rake it in.

Sow more thinly than instinct suggests. A packet holds a great many seeds and the plants mound out to eight or ten inches (20 to 25 cm) across, so a heavy sowing gives you a crowded mat that goes mildewed and rots at the base in the first wet spell. Aim to end up with plants six to eight inches (15 to 20 cm) apart after thinning. Keep the surface damp with a fine spray until the seedlings are an inch (2.5 cm) tall, which usually means a light watering once a day in warm weather.

For a continuous show, sow a short strip every three weeks from mid spring until about two months before your first expected frost. Each sowing peaks for six to eight weeks and then gets tired, and staggered sowings mean something is always in its best flush without you having to nurse an exhausted planting.

Write down the date of each sowing strip and the date the first flowers opened on it. After one season you will know exactly how many weeks your conditions take, and you can time later sowings to land where you want them instead of guessing.

Sowing in trays for an early start

Start indoors only if you want plants in flower at the moment the frosts end, for the edge of a container or the front of a bed that has to look finished early. Sow six to eight weeks before your last frost date, which is much later than you would sow slow bedding; sow it in midwinter and you will have leggy, exhausted plants long before you can put them out.

Fill a tray with seed-starting mix, firm and level it, and water it thoroughly from below by standing it in an inch (2.5 cm) of water until the surface darkens. Scatter the seed across the surface, press it down with a flat board, and do not cover it. Leave the tray uncovered or use a propagator lid with the vents open, in a bright spot at 60 to 70F (15 to 21C). A sealed bag over the tray holds too much humidity for a seed that needs light and airflow, and invites damping off.

Seedlings appear within a week. Thin or transplant into cells at three or four true leaves, two or three seedlings to a cell, since a small clump makes a fuller plant and the seedlings are fiddly to separate cleanly. Grow them on cool, at 55 to 60F (13 to 16C), in the brightest place you have. Warm and dim gives you pale stretched seedlings that flop when they go outside.

Harden off over about ten days before planting out. Sweet alyssum takes this in its stride compared with tender bedding, and the plants can go out a week or so before your last frost date, since a light frost does no real damage to an established young plant.

1. Water the compost before you sow Stand the tray in shallow water until the surface goes dark, then drain it. Watering after sowing washes uncovered seed into corners and down into the compost where the light never reaches it.
2. Scatter thinly and press, do not cover Rub pinches of seed out from six inches (15 cm) up, then press the surface flat with a board. Firm contact with damp compost is what the seed needs; a layer of covering is what kills it.
3. Leave it uncovered and bright Keep the tray at 60 to 70F (15 to 21C) in good light with air moving over it. A closed propagator or a plastic bag traps humidity and brings on damping off in seedlings this small.
4. Transplant in twos and threes At three or four true leaves, lift small clumps rather than single seedlings and set two or three per cell. Clumps knit into one bushy plant and are far quicker to handle.
5. Grow on cool and bright Hold the young plants at 55 to 60F (13 to 16C) with as much light as possible. Cool growing produces the short, dense plants that flower heavily; warmth without light produces stretched stems that never recover.

Sowing into paving cracks, walls and gravel

One of the best uses for sweet alyssum is in the places you cannot plant properly. It roots into a crack between paving slabs, the joint in a dry stone wall or a strip of gravel beside a path, and it flowers there as well as it does in a bed, because those positions drain sharply and warm early, which is exactly what the plant likes.

Sow by brushing a pinch of seed into the crack with your finger and then watering it in with a can fitted with a fine rose. There is no need to add soil. The dust and grit that has collected in a paving joint holds more moisture than it looks as though it does, and the seedling roots run down into the cool damp underneath the slab where they are far better supplied than a plant in an open bed.

Keep the crack watered daily until the seedlings are established, because a shallow joint dries out fast in the first fortnight. After that these plants generally need no attention at all, and they will shed seed into the surrounding joints and come back on their own in following seasons.

In a gravel path, scratch the surface lightly with a hand fork before scattering the seed so it falls between the stones rather than sitting on top of them, then water it in. Do not sow gravel that is laid over landscape fabric, since there is nothing underneath for the roots to reach and the plants dry out and die at the first hot spell.