Delphinium Pests and Diseases: Slugs on the Spring Shoots, Mildew and Crown Rot

Slugs and snails: the one that actually kills plants

Put this first because it deserves to be first. A delphinium crown wakes early, when the soil is cold and damp and slugs are already active, and it produces a small number of thick, soft, sappy shoots that are among the most attractive things in the whole garden to a slug. There is no reserve of foliage to fall back on. Lose the shoots and you have lost the plant, and this is how most delphiniums that disappear actually die.

The damage is unmistakable once you know it: shoots grazed off cleanly at or just above soil level, sometimes leaving a chewed stump, sometimes nothing at all, with a silvery trail nearby on a dry morning. Half-eaten shoots that carry on growing turn into blind stems that never form a spike. All of it happens at night, and by daylight there is nothing to catch.

The window that matters is short. From the moment the crown shows movement until the shoots are eight to ten inches (20-25 cm) tall, the plant is defenseless; above that the stems firm up and the risk drops away. Everything you do should be concentrated in those few weeks, and it needs to be in place before the shoots appear, not after you notice damage.

Combine methods, because no single one is enough on its own. Clear the ground, put a barrier down, and go out at night with a torch on the first mild damp evenings. Hand-collecting for ten minutes on two or three consecutive nights at the start of the flush removes the individuals that are actually feeding on your plants, which no barrier can do.

1. Clear the crown before growth starts Rake away every dead leaf, old stem and lump of loose mulch within a hand's width of the crown. Slugs shelter in that litter by day and only have to travel a few inches at night. Bare, firm soil around the crown makes them cross open ground.
2. Lay a barrier as the shoots break Ring the crown with a band of sharp grit two inches (5 cm) wide and half an inch (1 cm) deep, or fit a copper collar. Renew grit after heavy rain, which washes it flat and lets slugs walk over it.
3. Patrol by torchlight on mild damp nights Go out an hour or two after dark on the first warm wet evenings of the shoot flush. Pick off what you find on and around the plants. Two or three nights running at the right moment is worth weeks of daytime effort.
4. Bait sparingly and precisely if you use it If you use pellets, scatter them thinly around the crown at the rate on the label rather than heaping them. A ring of a few pellets per plant works; a pile does not work better and is a hazard to wildlife and pets.
5. Stop when the stems are ten inches (25 cm) Once the shoots are firm and ten inches (25 cm) up, the danger has passed for the spring. Start again on the new shoots after you cut the plant back for its second flowering, when slug numbers are often higher.

Powdery mildew on crowded clumps

Powdery mildew shows as a dusty white or grayish coating on the upper surface of the leaves, usually starting on the lower foliage in the middle of the clump and working upward. Badly affected leaves yellow, curl and drop, and a plant that loses its lower half of foliage before flowering produces a visibly poorer spike.

It is a disease of still, humid air rather than of wet soil, which is why it arrives in the center of an unthinned clump first. Twelve stems packed together hold a pocket of damp air that never dries after rain or dew, and that is the infection site. Thinning to five or six stems in spring is the single most effective control, and it costs nothing.

The other two contributing causes are drought at the roots and soft nitrogen-fed growth. A plant with dry roots and humid leaves is the worst combination there is. Water deeply at the base and never over the foliage, water in the morning so leaves dry before nightfall, and feed balanced rather than high-nitrogen.

If it appears anyway, pick off the worst leaves and bin them rather than compost them. Late in the season it is often not worth treating at all: cut the plant back for its second flush and the regrowth comes clean. What matters is autumn hygiene, because the spores overwinter in the debris around the crown.

Crown rot, blackened stems and what is not worth saving

The rots are less common than slugs but they are the ones that end a plant outright. Crown rot shows as shoots that emerge and then wilt and collapse for no visible reason, with a soft brown or slimy crown when you scrape the soil away and an unpleasant smell. It is a problem of wet, heavy, badly drained ground and of crowns sitting in damp mulch, and it is far more likely over winter than in summer.

Blackening of the stem base, sometimes with the shoot toppling at that point, is the other pattern to know. On a delphinium, a stem that is black and soft at ground level while the top is still green is not recoverable; cut it out at the base and remove it from the bed rather than leaving it lying there.

Prevention is drainage and hygiene, not spraying. Plant on ground that does not lie wet in winter, working in grit and compost on heavy soil, and set the crown at soil level rather than in a dip that collects water. Keep mulch pulled back an inch or two so the crown itself is never buried in damp material. Cut the plant down in autumn and clear the debris away.

Know when to stop. If most of the crown is soft, the plant is not going to come back and keeping it in the ground risks the neighboring clumps. Lift it, remove the soil from that spot, and start a new plant from a cutting off a healthy clump elsewhere in the garden rather than replanting in the same place.

Handling the plant safely, and keeping records that shorten next year

Every part of a delphinium is poisonous if eaten, seed and young foliage most of all, and the sap irritates the skin of some people. This is a real consideration, not a footnote: wear gloves when thinning, cutting back, dividing or taking cuttings, keep the cut material out of reach of children and pets, and never dump the trimmings anywhere that livestock can graze. Wash your hands before eating or touching your face.

It also affects where you site the plant. A tall delphinium in the middle of a border is not a concern, but one leaning over the edge of a path where a small child walks, or over a fence into a paddock, is worth moving.

On the record-keeping side, three dates make next season far easier. Write down the date the first shoots broke ground, the date you first saw slug damage, and the position of each clump in the bed. The first two together tell you exactly how many days after emergence your barrier and patrols have to be in place, and that gap is remarkably consistent from year to year in one garden.

Add a note of which clumps got mildew and how many stems each was carrying. After two seasons you will be able to see the correlation in your own bed rather than taking it on trust, and you will know which positions in the garden are the damp, still ones to avoid.

Do not compost delphinium debris from a clump that had mildew or any rot. Both carry over in the litter around the crown, and returning that material to the bed as compost is the usual way an infection travels from one season to the next.