Heuchera Problems: Vine Weevil Grubs, Crown Rot and Scorched Leaves

Vine weevil: the pest that actually kills heucheras

Vine weevil grubs are the main killer of coral bells, and heuchera is one of their favorite hosts. The adult is a dull black beetle about a third of an inch (8 mm) long that cannot fly, feeds at night and lays eggs in the soil around the crown. The grubs are creamy white, C-shaped, legless, with a tan head, and up to half an inch (10-12 mm) long. They live in the soil and eat roots, then work upward and hollow out the woody crown from the inside.

The symptoms are unmistakable once you know them. The plant looks slightly dull, then wilts on a warm day and does not recover overnight, then a week later you tug the rosette and the entire top comes away in your hand with no roots at all. The base of the stem is brown and tunneled, sometimes with a grub still inside. Plants in containers and freshly bought plants are hit hardest, because the grubs travel in potting mix.

The other clue arrives months earlier and is easy to miss. Adult weevils chew small semicircular notches out of the edges of leaves, as though someone had gone round with a hole punch. That damage is cosmetic and harmless in itself, but it tells you adults are present and laying, and it gives you a season's warning before the grubs do real damage.

What to do when you find them

Act on the whole root ball, not just the plant. Grubs that are left in the soil will move on to the next thing you plant there.

1. Lift and wash Take the plant up whole and wash all the soil off the roots so you can see. Work over a bucket or a hard surface rather than over the bed, so grubs do not simply drop back into the ground while you work.
2. Collect every grub Go through the soil and the root ball with your fingers and pick out every grub you find. A badly affected plant may hold twenty or more. Destroy them; do not throw them on the compost pile, where they will happily continue.
3. Save the top as a cutting If the leafy rosette is still firm, cut the stem back above all the tunneled, brown tissue and treat it exactly like a crown cutting: pot it into fresh gritty potting mix with the cut stem buried to the base of the leaves, keep it shaded and damp, and it will root in four to eight weeks. Most heucheras that appear killed by weevil can be saved this way.
4. Do not reuse the compost Discard the old potting mix rather than tipping it into a bed, and scrub the pot before reusing it. Eggs and small grubs are almost invisible and survive happily in stored compost.
5. Treat the ground Water parasitic nematodes into moist soil once it is above 41F (5C), which for most gardens means late spring and again in early autumn. Two applications a year in the areas where heucheras grow will hold the population down. Nematodes need moist warm soil to work, so water the ground before and after applying.

Check every heuchera you buy before it goes in the ground. Knock it out of its pot and look at the root ball. Finding one grub at that point costs you five minutes; finding it a year later costs you the plant and infests the bed.

Crown rot, and telling it apart from weevil

The other way heucheras die is rot at the crown, and it happens for the opposite reason to drought: the plant sits in wet soil over winter, or debris and mulch have been piled against the leaf bases. The middle of the rosette turns brown and mushy, leaves pull away with a soft slimy base, and there is a sour smell. Weevil damage, by contrast, is dry and tunneled, and the tissue is brown but crumbly rather than wet.

Prevention is drainage and air. Plant on a slight rise on heavy soil, work grit and compost into the planting area, and keep mulch pulled back a couple of inches (5 cm) from the leaf bases rather than heaped over the crown. Remove dead leaves from around the plant in late winter so air can move through the base.

If rot has taken the center, cut back to firm tissue and take whatever sound side rosettes exist as cuttings. A crown that is soft all the way through is not recoverable. Do not replant another heuchera in exactly the same spot without improving the drainage first, or you will repeat the result.

Scorch, powdery mildew and minor troubles

Brown, papery patches on the upper surfaces of leaves, worst on the side facing the afternoon, are sun scorch rather than disease. Pale and light-colored foliage burns easily; dark purple and bronze foliage rarely does. The damage does not heal, so remove the affected leaves and either move the plant or give it afternoon shade. Scorch is also much worse on plants that dried out at the root, so it often signals a watering problem rather than too much light on its own.

Powdery mildew shows as a white dusty film on the leaves in humid weather, particularly on crowded plants and in still air. It disfigures rather than kills. Space plants so air moves between them, remove the worst leaves, and water the soil rather than the foliage. It usually clears once the weather changes.

Slugs and snails graze young leaves in spring and leave irregular holes with slime trails, but heuchera is not a favorite and the damage is rarely serious. The one exception is the flower stems, which slugs will climb and ring-bark, and freshly divided plants, whose soft new growth is vulnerable until the leaves harden.

Note in your records the date you first saw notched leaf edges, and which plants they appeared on. Vine weevil comes back to the same beds year after year, and knowing that adults were active by early summer last year tells you exactly when to apply nematodes this year rather than guessing.