Help! My plant’s leaves are curling, and they look black underneath. What is happening?

Sounds like aphids.

Curling leaves, yellow leaves, and black underneath the leaves are all classic signs of an aphid infestation. Don’t panic. This infestation can easily be controlled.

Aphids are tiny soft-bodied insect that feeds on plant juice. They are so small it is hard to see a single one just by looking, but if left unattended, they will quickly multiply and cause harm to the plant.

There are several species of aphids, including the bean aphid, cabbage, potato, green peach, melon, and woolly apple. Some of these stick with one plant; others will eat almost anything.

Each species will attack different parts of the plants, including leaves, stems, flowers, and fruit – even the roots. Aphids are fond of succulents and tender new growth. The nymphs, or young insects and adults will suck the nutrients from plants. 

You may notice a sticky wet texture on leaves and stems. This is another sign that aphids have been feeding on your plant.

This “honeydew” is a sugar-rich substance secreted by the insects as they feed on the plant sap. If not treated, honeydew can attract ants that feed on the sugar. This can also add to the development of sooty mold and make the leaves and branches appear black.

The first step once you notice aphids is to spray the plant with a jet stream of water. Blast those insects off your plant. This might work, but you can do more to get rid of the insects.

A spray bottle with a few drops of dishwashing liquid mixed with water is a good next step. The soap will suffocate the bugs, so make sure you spray everywhere you see clusters.

Equal parts rubbing alcohol and water is another solution. 

Be careful with this one. Test a few leaves and ensure the mixture does not damage the plant. Horticultural oil will work too, but keep your plant out of the sun when applying the oil.

I generally use a spray bottle of water and wipe each leaf after a good soaking. It is tedious, but it works.

I recently noticed a potted tree that lives inside during the winter but thrives being outside during the warmer months looking less than its usually vibrant self. The leaves were curling, and the stems looked dark. I flipped over the leaves, and sure enough, I found aphids. It’s taken a few days, but the plant has been wiped clean and is on the path to recovery.

If you are the think ahead type, companion plants can help manage aphids. Catnip will act as a repellant. So will garlic and chives.

Mustard and nasturtiums will attract aphids. Plant these near your favorite plants as a decoy and draw the insects away from other more valuable plantings.

The good bugs-such as ladybugs and lacewings, feed on aphids. Attract these to your garden with flowers, herbs, and a water source, and they will feast on the pesky aphids. Another bonus of the decoy plants, by attracting the aphids, they become a good food source for the ladybugs and other beneficial insects.

Accepting that aphids are both helpful by attracting the good bugs and harmful by feeding on tender new leaves gives gardeners the knowledge to manage the insects and create an environment that reflects a healthy garden.