Potted plants could be suffering from caterpillars

Q: I bought three pots just before Mother’s Day that have geraniums and cascading petunias in them. They bloomed great at first and I have kept the petunias dead-headed like you’ve always suggested. Lately, though, I’m finding small holes in the geranium buds and the petunias just before they open, sort of distorting them. Also, I’ve found small clusters of black stuff on some of the leaves. What is going on and can I fix it? — L.P., Albuquerque

A: To me it sounds like the geraniums and the petunias are home to some sort of caterpillar. I’ve always known it as bud worm.

I remember being taught that more than likely a moth laid her eggs on the plants and when they hatched, the young caterpillars would chew their way into the buds of the flowers. The distortion is caused to the petals by having the veins of the petals chewed up, not allowing them to grow and open as they usually would.

The deadheading, removing the spent and finished blooms, is a great thing to be doing because you might be removing some of the bugs along with those spent heads. Just make sure you are pitching those deadheads into the trash. Don’t leave them lying around.

The black stuff you’ve noticed is more than likely the caterpillar poop. Not harmful, just an eyesore for sure. The treatment is pretty simple. I suggest you go to a well-stocked nursery and ask for a spray that contains “BTk,” Bacillus thuringiensis kurstaki.

This spray is very effective, albeit a bit slow, and requires several sprays in succession in order to get the caterpillars. I do know that BTk does degrade in sunlight quite rapidly, so a fresh spraying every two to three days will give you the upper hand. Remember to spray at dusk when most beneficial insects have flown home.

Now the hard part, I will suggest that you pinch off or snip away any and all flower buds the plants are wearing before spraying. That way the bugs aren’t tucked away safely inside the buds and not affected by the BTk. Also, be sure to water the plant first and then apply the spray. As the caterpillars feed they get a really bad upset stomach, stop feeding eventually, and then die.

One more consideration about the BTk is that it does sadly affect butterfly caterpillars, so don’t go spraying everything in the garden, especially if you’re seeing true butterflies. Only spray the geranium and petunia pots.

After five to eight successive sprays the infestation should be under control and you can allow the plants to set a new series of bloom for you. It’s a bit of dedication I know, but you can get those pots blooming healthily for you for the rest of the growing season.

Happy Diggin’ In!

Tracey Fitzgibbon is a certified nurseryman. Send garden-related questions to Digging In, Albuquerque Journal, 7777 Jefferson St. NE, Albuquerque, N.M. 87109, or to features@abqjournal.com.