Gadenin日本語Get the appBrowning: heat, drought or rot
Brown patches in the middle of a sweet alyssum are by far the most common complaint, and there are three separate causes that need different responses. Diagnose before you act, because shearing a plant with root rot wastes two weeks and pulling out a heat-stressed plant throws away a full second flush.
Heat browning is dry and crisp, starts in the center of the mound where air moves least, and appears during or just after a hot spell while the outer growth is still green and flowering. The plant is not diseased. Shear it to half height, soak the root zone and feed lightly, and it rebuilds inside a fortnight.
Drought browning looks similar but follows a period without water, particularly in a container, and the whole plant is affected rather than just the center. Water thoroughly, standing a pot in a bucket for twenty minutes if the compost has shrunk away from the sides, then shear back and treat it as above. Sweet alyssum recovers from drying out better than most bedding, but not from drying out repeatedly.
Rot is the one that ends the plant. It is soft rather than crisp, sits right at the base of the stems where they meet the soil, sometimes smells sour, and the stems collapse when pinched instead of snapping cleanly. It follows a wet cool spell, a blocked drainage hole, planting too deep, or a saucer left full of water. There is no shearing your way out of it. Clear the drainage, stop watering until the surface dries, and if more than a third of the crown has gone soft, remove the plant before the problem reaches its neighbors.
| Symptom | Likely cause | Action |
|---|---|---|
| Crisp brown center, dry to touch | Heat and hot nights | Shear to half height, soak, feed lightly |
| Whole plant crisp and gray | Dried out in a container | Soak the root ball, then shear back |
| Soft brown base, stems collapse | Crown or root rot | Improve drainage; remove if widespread |
| Gray fuzz on stems in wet weather | Gray mold in congested growth | Thin or shear, improve airflow |
| Small round shot holes in leaves | Flea beetle | Keep watered; damage is cosmetic on mature plants |
| Sticky leaves, clusters on tips | Aphids | Spray off with water; reduce feeding |
| Ragged holes, silver trails | Slugs and snails | Protect seedlings; check under the mound after rain |
| Yellow lower leaves, soggy soil | Waterlogging | Clear drainage holes and stop watering |
| Seedlings toppling at soil level | Damping off | Uncover the tray, cut watering, more light |
Rot at the base, and how to avoid it
Sweet alyssum is genuinely drought tolerant and has almost no tolerance for standing water. Nearly every case of a healthy plant dying suddenly in midseason is rot at the crown, and it is almost always a drainage problem rather than a disease that arrived from outside.
The risk factors are consistent: a container without an open drainage hole, a decorative pot with no hole at all used as a cachepot, a saucer left full after rain, a heavy clay soil that stays wet for days, planting the crown half an inch (12 mm) too deep, or a dense mat of overcrowded plants that never dries out after rain. Any of those on their own will do it in a wet spell.
The prevention is straightforward. Plant at the same depth the plant was growing, never deeper. Add grit to the planting hole on heavy soil, or plant on a slight ridge so water runs away from the crown. Use containers with real drainage and tip saucers out after rain. Space plants at six to eight inches (15 to 20 cm) so air moves between them once they mound up. And in a wet cool spell, simply stop watering.
Once rot is established in a plant there is no treatment. Take it out along with the soil immediately around it, and do not replant sweet alyssum in exactly the same spot in the same season without improving the drainage first.
Insects: aphids, flea beetle and slugs
Aphids gather on the soft growing tips and under young leaves, leaving the foliage sticky and sometimes distorted. On a plant this dense they can build up unnoticed. A firm spray of water over the plant knocks most of them off, repeated every few days for a fortnight, and that is usually the whole treatment. The underlying cause is almost always overfeeding, since soft nitrogen-rich growth is what attracts them, so cut the feed rate rather than reaching for a spray. Note also that alyssum is grown specifically to attract hoverflies whose larvae eat aphids, so a broad insecticide on this plant works directly against the reason for having it.
Flea beetle is the pest that surprises people, because sweet alyssum belongs to the same family as cabbages and rocket. Tiny beetles that jump when disturbed chew small round shot holes through the leaves, worst in dry weather and worst on seedlings. On a mature flowering plant the damage is cosmetic and can be ignored. On a newly emerged sowing it can strip the seedlings entirely, so keep the surface damp during germination, since flea beetle activity drops sharply in moist conditions, and sow a little more thickly than you need where you know the pest is present.
Slugs and snails take newly emerged seedlings overnight, and at that size a single slug clears a patch rather than nibbling it. The vulnerable window is the first three weeks. It is also worth remembering that a dense established alyssum mound is a cool damp shelter, so if you have planted it as an edging beside vegetable seedlings, check underneath it after rain rather than assuming the slugs are coming from somewhere else.
Keep a note of where and when each problem first appeared. Rot and mildew tend to recur in the same badly drained corner year after year, and a two-season record identifies that position far faster than trying to remember which patch failed.
When to shear and when to give up
The general rule is that if the growth is green anywhere and the base is firm when you squeeze it, the plant is worth saving and a shear plus a soak will bring it back within a fortnight. Sweet alyssum regenerates so readily from leafy stems that it is nearly always worth trying before removing anything.
Give up when the base has gone soft, when the crown is bare and woody with only a few live strands, or when the plant has been dried to collapse several times over a few weeks. In each of those cases it will not regain density this season, and leaving it in place keeps a source of rot next to healthy plants.
Replacing it is easy in a way that most bedding is not. Sweet alyssum takes about six weeks from sowing to flower, so a fresh sowing scattered on the surface of the gap will be flowering within six or seven weeks, which is usually quicker than nursing a failed plant back to anything worth looking at. Where a whole planting has failed at once, look at the position and the drainage rather than the plants, because the same spot will do the same thing again next year.