Horse nettle Beware the Wild Perennial Tomato
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----Maizie in Lewes, Delaware
A. Yes; the common name of this plant is "horse nettle", but it looks like a spiny eggplant when young and a yellow cherry tomato when mature—which makes sense as it's in the tomato/potato/eggplant family. The flowers look like eggplants; the leaves smell like potatoes when you crush them; and the fruits look like little yellow tomatoes—but they're poisonous. It also has the nasty spines she mentioned; hence the botanically incorrect but helpfully informative common name that includes the warning word 'nettle'. And it's perennial; in fact, its underground rhizomes are said to be as tenacious as running bamboo.
So: where did this alien invasive monster come from?
The Carolinas. It's native to the American South and one of a surprising number of poisonous plants in the tomato family found in the US, including Jimsonweed (aka Angel's Trumpet, which is what you'll hear if you eat any part of it); black henbane, which has gorgeous flowers but can be fatal to farm animals when its growing in fields cut for hay; and the deadly nightshade sisters—bittersweet and black.
But the same family also includes peppers, tobacco and the famed Nicandra, or "Apple of Peru"—a tomato-like plant that has a long folk history of repelling flies. Nicandra isn't poisonous, but it is considered an invasive weed—although many farmers swear by its ability to keep flies away. Or maybe just kill them. Nobody knows for sure.
So what does our listener need to do about her weird weed?
Rule Number One: Don't plant any yellow cherry tomatoes nearby if you're easily confused!
Seriously, the bitter taste would make you spit this fruit out instantly if you did try one. Now, all of the literature on this plant agrees that herbicides are useless. And cultivation, especially with a tiller, would make the problem much worse as it would cut up and then replant thousands of the rhizomes. (That's how they deliberately plant horseradish. They just till the field after harvest and all the little root bits they missed get chopped up and replanted for the following season.)
All of which means that it's time for my 'rope a dope' tough-weed technique.
In Year One, cut down the plants as soon as you can ID them. Force the roots to expend energy growing new leaves but try not to allow any of those leaves to reach the size where they can achieve photosynthesis. You'll eventually starve the plant, which is used to being left alone to spread via its rhizomes and by dropped seed. (You could use a weed whacker to keep up with the job; but only if you make sure to always wear good shoes, long pants and such. You want to keep the toxic juice and nasty spines offa your bare skin.)
In Year Two I would continue to cut the plants back until we get to a hot dry stretch in the summer, and then use a flame weeder to toast newly emerging baby plants; or spray them with a non-toxic herbicide like herbicidal soap or one of the newer iron-based broadleaf herbicides. But you have to knock the plants down for a full season first so that the new sprouts will emerge small and weak enough to be vulnerable to these tactics..
Another alternative is to solarize the soil. Keep cutting them back now. Then scalp the area super low in the spring, really saturate it with water and cover it tightly with one or two mil thick clear plastic, as we describe in the soil solarization article in the A to Z Gardens Answers section of our website. It'll take a full growing season in Delaware, but the underground rhizomes will be really most sincerely dead by Fall if you follow the directions carefully.
And finally: is a Delaware gardener correct to try and find advice that's more local than PA's Extension service?
No. Her conditions are essentially the same as in PA; heck—you can step into Delaware from parts of PA. What you don't want to do is take advice from a state that has wildly different growing conditions—like California or the Dakotas.