Sweet Alyssum Care: Watering, Feeding and the Midsummer Shear Back

It asks for less than most bedding plants

Sweet alyssum is one of the few annuals that is genuinely happier on the lean side. It is a plant of dry, stony, open ground, and it flowers hardest when it is growing in free-draining soil with modest fertility and full sun. Rich soil, heavy feeding and constant moisture produce a lush green mound with very few flowers, which is the opposite of what most people are growing it for.

In the ground, established plants in reasonable soil need watering only in a genuine dry spell, and a good soak once a week is more use than a daily splash. The roots go deeper than the low mounded shape suggests, and watering deeply encourages them down where the soil stays damp. In containers the calculation is different simply because the volume of compost is small: check the top inch (2.5 cm) and water thoroughly when it is dry, which in high summer usually means daily for a small pot and every second or third day for a large one.

The one thing to avoid is water standing around the crown. Sweet alyssum tolerates drought easily and rots readily. A container without free drainage, a saucer left full of water, or a heavy soil that stays wet after rain will kill a healthy plant from the base upward in a matter of days. If you are in doubt about your soil, plant it on a slight ridge or add grit to the planting hole rather than watering less.

Feeding: why less gives you more flower

If you have improved a bed with compost, sweet alyssum planted into it needs no feeding at all for the whole season. In containers, where every watering flushes nutrients out of the bottom, a balanced liquid feed at half the label strength once a month is plenty. That is the entire feeding regime, and pushing beyond it makes the plant worse rather than better.

The failure mode is easy to recognize. A heavily fed plant grows tall, loose and dark green, the mound opens up in the middle and falls apart under its own weight, and the flowers become sparse and scattered instead of covering the whole surface. It also becomes noticeably more attractive to aphids and much more likely to rot at the base after rain. If your plants look leafy and flower poorly, the answer is to stop feeding entirely for a month and shear them back, not to switch to a high potash product.

Sun matters more than feed for flowering. Sweet alyssum wants at least six hours of direct sun a day, and in less than that it goes leggy, produces perhaps a third of the flowers, and sprawls rather than mounding. In a very hot climate, light afternoon shade actually helps by keeping the plant flowering longer through the worst of the season, but that is shade from extreme heat, not shade in general.

Note which of your positions kept flowering through the hottest fortnight and which stalled. Sweet alyssum responds so directly to heat and drainage that a record of where it did well in your own garden is worth more than any general recommendation about sun and soil.

The midsummer stall, and the shear that fixes it

Part way through summer a sweet alyssum that has been flowering solidly for weeks will thin out, brown in the middle, stop producing new flowers and generally look finished. Most people conclude that the plant has died or has a disease and pull it out. It has done neither. It is a cool-season plant that flowers hardest with warm days and cool nights, and when night temperatures stay above about 70F (21C) for a run of days it shuts down flowering and lets the older growth die back.

The fix is a hard shear and it is dramatic in its effect. Cut the whole plant back to about half its height with scissors or one-handed shears, working evenly across the mound and taking browned and green growth together. Do not pick through it trying to save the good parts; an even cut is faster and produces a far better shape. Then soak the root zone thoroughly and give a balanced liquid feed at half strength.

The plant looks like a shorn stump for about five days. New shoots then break from every cut stem and from the base, and within ten days to two weeks you have a fresh, dense mound covered in buds again. That second flush usually carries right through to the first frosts, and in a mild autumn sweet alyssum is often the last thing still flowering in the garden. This is the single most useful piece of knowledge about the plant: it is the difference between an annual that is over by midsummer and one that performs for five months.

Time the cut for the point where the worst heat is breaking rather than in the middle of it, and always water heavily before or at the same time as you cut. Shearing a plant sitting in bone dry soil produces no regrowth at all, because there is no moisture available to drive it, and that is the most common reason the technique appears not to work.

1. Soak the ground first Water heavily the day before cutting, or immediately after. Regrowth is driven entirely by moisture, and a sheared plant in dry soil sits as a stubble for weeks instead of breaking new shoots in five days.
2. Cut to half height, evenly Shear straight across the mound with scissors, taking about half of everything including green growth. Selective trimming of only the brown parts takes an hour and leaves a lopsided plant.
3. Leave leafy stem below the cut Keep at least an inch or two (2.5 to 5 cm) of leaf-bearing stem on every branch. Sweet alyssum regrows readily from leafy tissue and poorly from bare woody stems at the crown.
4. Feed at half strength A balanced liquid feed at half the label rate gives the regrowth what it needs. Full strength on a plant with most of its leaf removed scorches the fine roots at the worst moment.
5. Keep it damp for two weeks Check the soil every day or two through the recovery period. One dry spell during regrowth turns a full second flush into a thin, patchy one that never fills out.
6. Stop shearing six weeks before frost After that point there is not enough warmth and daylight left to rebuild and flower, so a hard cut leaves you looking at stubble for the rest of the season. A light tidy is fine at any time.

Growing it at the edge of a vegetable bed

Sweet alyssum is worth growing at the edge of a vegetable bed for reasons that have nothing to do with how it looks. The flowers are tiny, shallow and open, which suits the short mouthparts of hoverflies, parasitic wasps and other small beneficial insects that cannot feed at deeper flowers. Those insects work an alyssum edging constantly through the day, and their larvae are among the most effective aphid predators in the garden.

Plant it along the sunny side of a bed, in a strip a foot (30 cm) wide, or in blocks at the corners and at intervals along the middle of a long row. Spacing the plants through the crop rather than only around the perimeter matters, because the beneficial insects that emerge do not travel far, and a hoverfly feeding at the end of a thirty foot row does little for aphids at the other end.

For this to work the plants have to be in flower when the pests arrive, which means sowing at the same time as the crop rather than after the problem starts. Then keep them flowering with the mid-season shear, staggering the cuts so that half the strip is always in flower rather than shearing the whole edging at once and leaving the bed without any nectar for two weeks.

It takes practically no room, tolerates being walked past and brushed against, and it will not compete with the crop, since it is shallow rooted and low. The one thing to watch is that a dense alyssum strip gives slugs somewhere damp and sheltered to sit next to your seedlings, so check underneath it after rain.