Gadenin日本語Get the appSlugs and the emerging shoot: the one that actually kills plants
The most dangerous fortnight in a dahlia's life is the one where the first shoot pushes through the soil. That shoot is thick, soft, full of water and is the only thing above ground for yards in an otherwise bare spring bed. Slugs and snails find it immediately. A shoot can be grazed to a stump overnight, and while a strong tuber will often push a second shoot, a weaker one will not, and you lose the plant without ever knowing why.
This is why the bare patch of soil where you know a tuber is planted deserves your attention every day from planting until the shoots are 6 in (15 cm) tall and hardening. After that, the stems toughen and the damage becomes cosmetic holes in the leaves rather than a lost plant.
Go out after dark with a torch for the first week or two. That is when they feed and it is by far the most effective control there is, because you remove the individuals that are actually on your plants rather than hoping a barrier turns them away. Clear boards, pots, dense weeds and loose mulch from within 12 in (30 cm) of each crown, because that damp cover is where they spend the day.
- Clear the daytime shelter around each planting spot Move pots, boards, stones and thick weed cover away from the crown. Keep any mulch pulled back an inch from the stem until the shoot has hardened.
- Patrol after dark with a torch for the first two weeks Check from the moment the shoot breaks the surface. Ten minutes a night through the vulnerable fortnight saves more plants than anything else you can do.
- Ring the shoot with a physical barrier Grit, wool pellets or a copper ring around each shoot. Barriers work only while the shoot cannot be reached by leaves bridging over them, which is fine at this stage.
- Use a collar on the most exposed plants A plastic bottle with the base and cap cut off, pushed 1 in (2 cm) into the soil around the shoot, is a cheap and highly effective sleeve until the plant outgrows it.
- Start tubers in pots if slug pressure is severe A plant set out at 8 to 10 in (20 to 25 cm) tall with hardened stems skips the entire vulnerable phase. On a wet site this is the reliable answer.
If a shoot vanishes, do not dig the tuber up. Give it three weeks. Many tubers push a second shoot from a different eye, and digging around to investigate damages the roots of a plant that was going to recover.
Earwigs eat the flowers, at night, and hide by day
Ragged holes chewed out of the petals of an open dahlia, appearing overnight with nothing visible on the flower in daylight, are earwigs. They feed after dark and spend the day pressed into a dark crevice, very often inside the packed center of the same flower they have been eating. Shake an affected bloom over a sheet of white paper in the morning and they will drop out.
They are not a disaster for the plant and they eat aphids as well, but on a flower you intended to look at or cut, the damage is total. Control is a trapping job rather than a spraying job, because a spray that reaches an open dahlia flower is also reaching the pollinators working it.
Set traps before the first flowers open, not after the damage starts. An upturned pot loosely stuffed with straw or shredded newspaper on the top of each stake is the classic method and it works because it is a better hiding place than the flower. Empty the traps every morning well away from the bed. A rolled up piece of corrugated cardboard tied to the stem does the same job. Keeping the ground under the plants clear of debris removes the alternative shelter and pushes them into the traps.
| Symptom | Likely cause | First response |
|---|---|---|
| Shoot grazed to a stump overnight in spring | Slugs and snails | Night patrol with a torch, clear cover, add a collar |
| Ragged holes in open petals, nothing visible by day | Earwigs | Straw filled pot traps on the stakes, emptied each morning |
| Sticky distorted growing tips and buds | Aphids | Squash the colonies, hose the tips hard, leave predators |
| Pale stippling and fine webbing in hot dry weather | Spider mites | Hose leaf undersides twice a week, starting early |
| White floury coating on upper leaves in late summer | Powdery mildew | Water the soil only, thin crowded stems, strip low leaves |
| Buds that dry up and never open | Thrips or drought at bud set | Open a bud to check, remove affected buds, water deeply |
| Yellow mottling with stunted twisted growth | Virus, spread by aphids | Pull and destroy the plant, tuber included. No cure |
| Tuber soft and brown in storage | Rot from damage or damp storage | Cut back to clean tissue or discard, dry the rest harder |
The rest of the season, and the one problem with no cure
Aphids cluster on the soft growing tips and around the buds, distorting flowers as they open and leaving sticky honeydew that goes sooty black. Squash the first colonies with your fingers and hose the tips hard; hoverflies and ladybirds usually take over from there. They matter beyond the direct damage because they carry viruses from plant to plant.
Powdery mildew turns up on the lower leaves in late summer as a white floury coating, spreading upward. It rarely kills a dahlia but it strips the lower foliage and shrinks the flowers. Spacing, watering the soil rather than the leaves, and removing the bottom 6 in (15 cm) of foliage once plants are established push it back by weeks. Spider mites are the hot dry weather counterpart, showing as pale stippling then bronzing then faint webbing, and hosing the leaf undersides hard twice a week breaks the cycle if you start early.
Virus is the one you cannot fix. Yellow mottling or ring patterns on the leaves, stunted growth and twisted distorted new shoots on a plant that is otherwise well cared for means the plant is infected, and it will not recover. Because it is carried in the tuber, saving that tuber spreads the problem into next year and into any plants it sits next to in storage. Pull the whole plant, tuber included, take it out of the garden, and control the aphids on everything else.
Write down the date you first see slug damage, the date earwig holes appear, and which plants were worst. Both problems recur in the same corners of the same beds every year, and that record is what lets you set the traps and the barriers a fortnight before you need them.