Gadenin日本語Get the appVine weevil: the plant lifts free of the soil
This is the one that kills primulas, and it kills them without warning. A plant looks slightly dull, then wilts and does not recover after watering, and when you take hold of it to see what is wrong the whole rosette comes away in your hand with no roots on it at all. That is the diagnosis. Nothing else does it: not drought, not rot, not disease. The roots have been eaten off below the crown by vine weevil grubs.
The grubs are creamy white, about a third of an inch (8 mm) long, curved like a comma with a brown head and no legs. Tip the pot or the soil out over a sheet and you will usually find several, sometimes a dozen, in the top few inches around the root ball. They are worst in containers, where a single female's eggs can wipe out a pot, and they feed hardest through autumn and winter when nothing else appears to be happening.
The adults are dull black beetles about a third of an inch (8 mm) long that cannot fly, feed at night and cut characteristic notches out of leaf margins, as if someone had worked round the edge with a hole punch. Those notches are cosmetic, but they are the early warning that eggs are being laid, and they appear months before the roots go. If you see notched leaves in summer, act on the pots that autumn rather than waiting.
Treatment is biological rather than chemical for most gardeners: a drench of parasitic nematodes applied to damp compost when the soil is above 41F (5C), which is normally late summer to mid autumn and again in spring. Nematodes only work on the grubs, not the adults, so the timing matters. Alongside that, tip out and check any pot that has lost a plant, crush the grubs you find, and do not reuse the compost around other susceptible plants.
- Test any wilting plant by lifting it Take the plant gently by the base of the leaves and pull upwards. A healthy plant resists. One with no root resistance at all, coming away as a bare rosette, has been eaten and cannot be saved as it stands.
- Tip out the pot and search the compost Empty it onto a sheet or a path and break the root ball apart with your fingers. Look for curved legless white grubs with brown heads. Destroy every one you find and do not put that compost back around other plants.
- Salvage the top if there is any live crown If the crown itself is firm, trim off damaged tissue, set it in fresh damp compost in a cool shaded place and keep it just moist. Some rosettes root again from the base within a few weeks; badly chewed ones will not.
- Drench the remaining pots with nematodes Water the compost first, apply the nematode drench at the rate on the pack while the compost is above 41F (5C), and keep the pots damp for two weeks afterwards. Repeat in spring where weevil has been a problem.
Slugs and snails take the flowers first
Primulas are grown in exactly the conditions slugs and snails like: cool, shaded and damp. Damage appears overnight, and it starts with the flowers rather than the leaves, so a plant that was in bud yesterday is a set of chewed stumps this morning. Leaf damage shows as irregular holes in the middle of the leaf with ragged edges, quite unlike the neat marginal notches of the vine weevil adult, and there are usually silvery slime trails on the soil or the pot rim.
Check with a torch an hour after dark, which is when they are actually on the plant. Daytime searching under pots, stones and the leaf litter around the crowns finds where they shelter. Clearing dense wet debris from immediately around each clump, while keeping the mulch a little way out, removes the hiding places without taking away the moisture the plant needs.
Barriers work better around a small number of prized clumps than across a whole border. Copper tape round a pot, coarse grit in a ring around a new division, or simply raising pots on feet away from a damp path all cut damage substantially. Where you use pellets, use them sparingly and early, before the flower buds open, because the damage is done in the first two or three nights of the display.
Aphids, red spider mite and the sticky leaf
Aphids cluster in the middle of the rosette and on the flower stalks, and they favor plants growing under cover or indoors. The first sign is usually stickiness on the leaves rather than the insects themselves, followed by distorted flower stalks and buds that open lopsided. A firm jet of water on an outdoor plant knocks most of them off; indoors, wipe the crown out with a damp cloth and repeat after four days, because eggs and hidden individuals repopulate the plant on about that cycle.
Red spider mite is a warm dry weather problem and therefore a symptom of the wrong conditions as much as a pest in itself. The leaves go dull and finely speckled with pale dots, and in bad cases there is fine webbing between the leaf stalks. It hardly ever troubles a primula in cool damp shade. If you have it, the position is too hot and too dry, and moving the plant does more than any spray.
Gray mold appears on the flowers and the older leaves in cold, still, wet conditions as a fuzzy gray growth on tissue that has already collapsed. Pick off every affected leaf and flower, thin out overcrowded plants to let air move between them, and stop watering over the top of the crown.
| Symptom | Cause | First action |
|---|---|---|
| Plant lifts out with no roots | Vine weevil grubs | Search compost for grubs; nematode drench on remaining pots |
| Neat notches cut round leaf edges | Adult vine weevil feeding | Treat the compost for grubs that autumn |
| Flowers stripped overnight, slime trails | Slugs and snails | Check by torchlight after dark; barrier or bait early |
| Sticky leaves, distorted flower stalks | Aphids | Wash off or wipe out the crown; repeat after four days |
| Dull finely speckled leaves, webbing | Red spider mite in hot dry air | Move to cooler, damper shade |
| Gray fuzz on collapsed flowers | Gray mold in cold wet still air | Remove affected parts; improve air movement |
| Brown crisp leaf margins | Sun scorch or dryness at the root | Shade from midday sun; soak and mulch |
| Crown black and soft, leaves peel away | Crown rot from water sitting in the center | Usually fatal; water at the soil, not over the plant |
Record the date you first see notched leaves, the first slug damage and the first wilting plant, along with which pots were affected. Vine weevil returns to the same containers year after year, and last year's notes tell you which pots to drench before anything visible goes wrong.
Brown leaves that are not a pest at all
Most brown on a primula leaf is environmental. Crisp brown margins working inwards from the edge, on a plant in a bright position, is scorch from direct sun or from drying wind, and the answer is shade from midday onwards rather than any treatment. The same symptom on a plant in shade means the root ball has been dry, often because watering has only ever wetted the surface.
Yellowing of the oldest outer leaves while the center stays green and firm is normal turnover, especially after flowering and again in autumn. Pull those leaves away cleanly so they do not rot against the crown. Yellowing that starts at the center, or that takes the whole plant at once, is a root problem and means lifting the plant to look.
Pale, washed out leaves across the whole plant in a pot that has been in the same compost for a year are simple hunger. A balanced liquid feed at half strength every two weeks through the growing season fixes it in a month, and repotting or planting out fixes it permanently.