Pentas Problems: Spider Mites, Aphids and Yellow Leaves in Hot Weather

Spider mites: the one that actually matters

Two-spotted spider mites are the pest that ruins pentas, and they build fastest in exactly the weather the plant enjoys: hot, dry, still air, often against a wall or on a sheltered patio. They breed from egg to adult in about a week above 85F (29C), so a population that is invisible on a Monday can have stippled every leaf on the plant within two weeks. Container plants and plants under an overhang that never gets rained on are hit first and hardest.

Catch them by the look of the leaf, not by looking for the mite. The first sign is a fine pale stippling across the upper leaf surface, as if someone had touched it with a very fine sandpaper. Held up to the light the leaf looks freckled. Turn it over and you will see faint webbing in the vein junctions and, with a hand lens, moving specks the size of ground pepper. By the time webbing is obvious on the growing tips, you are already well behind.

Treat with water first. A hard spray from a hose aimed upward at the undersides of the leaves, repeated every three days for two weeks, knocks the population down dramatically because mites cannot cope with being wet and physically dislodged. Do it in the morning so the foliage dries. If that is not enough, use insecticidal soap or a horticultural oil, again on the undersides, and repeat at five to seven day intervals for three applications to catch the eggs as they hatch. Never use a broad-spectrum insecticide: it kills the predatory mites that were keeping the numbers down and the outbreak comes back worse, and pentas is usually covered in butterflies.

Prevention is mostly humidity and rain. Plants that get hosed down or rained on regularly rarely get mites; plants in a hot dry sheltered corner get them nearly every year. If you have a mite-prone spot, put something else there and grow the pentas in the open.

Aphids, whitefly and caterpillars

Aphids cluster on the soft new shoots and the flower stalks, particularly on a plant that has been fed heavily with nitrogen. They distort the new growth and leave sticky honeydew that turns black with sooty mold. Small colonies can be squashed between finger and thumb or washed off with water. Larger ones respond to insecticidal soap. Before you reach for anything, look for ladybird larvae and lacewing eggs on the same shoots; if they are present, do nothing for a week and they will often finish the job.

Whitefly show as tiny white triangles that lift off in a cloud when you brush the plant, and they live on the leaf undersides. They are a nuisance rather than a killer outdoors but multiply badly on plants overwintered indoors. Yellow sticky traps and a weekly rinse of the undersides keep them in check.

Caterpillars chew ragged holes in the leaves. Before you remove one, work out what it is. Pentas attracts butterflies for the nectar rather than as a larval host, so a caterpillar on it is usually a generalist and can be picked off. But if you are growing it as part of a pollinator planting, a few chewed leaves are the price of the show and the plant will outgrow the damage.

Yellow leaves: three different problems that look alike

Yellow leaves on pentas mean one of three things, and the pattern tells you which. Yellowing that starts on the oldest lower leaves and works upward, with the whole leaf going uniformly yellow before it drops, is normally a watering problem: either the plant has been dried out to wilting more than once, or it is sitting in soil that never drains. Check the soil two inches (5 cm) down and look at the drainage holes before doing anything else.

Yellowing on the newest leaves at the tips of the shoots, where the leaf blade goes pale yellow but the veins stay a clear green, is iron chlorosis. It happens in alkaline soil and in tap water with high lime, and it is common in container plants after a couple of months. Treat with a chelated iron product applied as a soil drench, and mulch with compost or pine bark to bring the pH down slowly over the season.

Uniform pale yellow-green across the whole plant with weak thin growth is simple starvation, most often in a pot whose slow-release fertilizer ran out midsummer. Give a balanced liquid feed at full strength once, then return to a half-strength feed every fortnight.

Root rot and knowing when to give up

The one thing that kills pentas outright is wet feet. Root and crown rot show as a sudden collapse: the plant wilts on a day when the soil is clearly damp, does not recover overnight, and the stem at soil level is brown, soft and easily crushed between the fingers. There is no treatment once it has reached the crown. Pull the plant, do not compost it, and fix the drainage before you replant that spot.

The judgement call is worth making early. A pentas that has been badly mite-damaged in midsummer, with more than half its leaves stippled bronze, will not recover into a good plant that season even if you clear the mites. If it is an annual planting, pull it, dispose of the top growth away from your other plants, and replace it. If it is a plant you intended to overwinter, take clean cuttings from any unaffected shoot tips, wash them, and start again from those rather than carrying mites indoors with the parent.

Anything you bring indoors for winter brings its pests with it into a warm room with no predators. Inspect leaf undersides with a hand lens and hose the whole plant down twice, a week apart, before it crosses the threshold.