Cosmos Problems: No Flowers, Flopping Stems, Slugs and Mildew

All leaf and no flowers

This is the defining cosmos problem and it is worth naming plainly: a six foot plant covered in beautiful ferny foliage and carrying almost no flowers is a fed and watered plant, not a sick one. Rich soil, manure dug into the bed, fresh all-purpose potting mix in a container, or a routine liquid feed all produce it. The plant is doing exactly what its genetics tell it to do on fertile ground.

The secondary cause is shade. Under six hours of sun the plant stretches and flowering falls away sharply, and if the soil is also rich the two problems compound into a plant that never flowers at all.

The fix is the same in both cases and it works. Stop feeding completely, cut every stem back by a third to just above a leaf joint, and water only in genuine drought. Buds usually appear about three weeks later. If shade is the cause and the plant is in the ground, it is worth lifting and moving it, because cosmos transplants tolerably well if you take a deep rootball and cut the top back at the same time.

Next season, choose the leanest sunny spot you have and put nothing into it. A cosmos bed is the one place in the garden where doing less produces a better result, and it is genuinely difficult to make yourself leave good ground alone.

Flopping and broken stems

Tall cosmos on hollow brittle stems goes over in wind and heavy rain, and once flattened it does not stand back up. The stems kink at the base and the plant spends the rest of the season flowering sideways across whatever is next to it. Almost every case is preventable and the prevention has to happen early.

Three things cause it. No support, or support put in too late. Soft growth from feeding and shade. And plants left unpinched, so all the weight sits at the top of one stem.

Set netting or a string grid at about eighteen inches (45 cm) when the plants are a foot tall, pinch once, and feed nothing. A plant grown that way frequently needs no further help at all. If a stem has already kinked, cut it back to the nearest sound leaf joint rather than trying to splint it upright; it will break again at the same point.

Slugs on young plants, aphids on the buds

Cosmos seedlings and freshly planted transplants are extremely attractive to slugs and snails. A healthy plant at dusk can be a chewed stub by morning, and because the whole seedling disappears, gardeners often assume it was never planted properly. The giveaway is a clean angled cut at soil level and a silvery trail on the soil in early light.

The risk window is the first two or three weeks after planting out or thinning, and it is worst in mild damp weather. Once the plant has a firm base and is a foot tall it is too tough to be destroyed overnight. Clear mulch and weeds a hand's width back from each stem, water in the morning rather than the evening, and go out with a torch an hour after dark for the first two nights. Hand collecting on two nights removes most of the local population from a small bed.

Aphids cluster on the soft tips and around unopened buds and distort the flowers as they open. On cosmos they rarely reach damaging numbers before hoverflies and ladybirds find them, and cosmos is an outstanding plant for attracting exactly those predators. Squash the first colonies by hand, then leave it a week before doing anything more.

1. Plant out larger Hold plants in pots until they have a firm base and stand six to eight inches tall. A bigger plant simply passes through the vulnerable stage before the slugs find it.
2. Clear cover from around the stems Pull mulch, fallen leaves and weeds back a hand's width from every plant. Denying slugs damp daytime cover within reach does more than anything you sprinkle.
3. Water in the morning for the first fortnight Evening watering leaves the soil damp through the hours when slugs feed. Switching to mornings for two weeks after planting makes a visible difference.
4. Leave the aphids a week Cosmos flowers pull in hoverflies whose larvae eat aphids in numbers. Reaching for a spray in the first week removes the predators along with the pest.

Powdery mildew and stem rot

Powdery mildew shows as a white dusty film on the upper leaf surfaces, starting low on the plant in late summer and working upwards. It is driven by warm days, cool humid nights and poor air movement, and a stand of cosmos planted too close together at the wrong spacing gets it every year in the same place.

Strip the affected lower leaves and remove them from the bed rather than dropping them on the soil. Thin out crossing stems to open the center of the plant. Stop watering over the top of the foliage. Cosmos tolerates losing its bottom leaves without much loss of flowering, so a firm strip is the right response rather than a cautious one. Next season, space at the full recommended distance, which is the real preventative.

Stem and collar rot is the other disease worth knowing. A plant that collapses at the base in an afternoon with no other warning has rotted at the soil line, and the cause is soil that stayed wet. There is no treatment. Pull it, improve the drainage or grow on a ridge, and do not replant cosmos in the same spot that season.

Write down the date you first see each problem and the bed it happened in. Slug damage, aphid build up and mildew all recur in the same places at roughly the same point in the season, and one year of dates lets you act a week before it starts.