Long Island scientist helps identify readers' garden weeds

Summertime! While reveling in tomatoes and black-eyed Susans, many Long Islanders are painfully aware we’re also smack in the middle of weed season.

Crabgrass, dandelions, lamb’s quarters and plantain are ubiquitous. Some of us hunt them down like wild dogs (just an expression — I love dogs!) and some simply accept them as part of life in the lawn.

But every now and then, something seemingly foreign pops up, and we don’t know what it is, let alone what to do about it. These readers found lesser-known weeds in their gardens this year, and Andy Senesac, a weed scientist with the Suffolk County Cornell Cooperative Extension in Riverhead, has stepped in to help identify them.

“This popped up this year, and have no idea what it is,” wrote Rita Libasci from Hicksville. “It starts off blooming with a tiny yellow flower, dries up quickly and then produces a white puff.”

What you have, Rita, is a weed called yellow goatsbeard (Tragopogon dubious) “a biennial cousin of dandelion, as you probably guessed from the seed head,” said Senesac, recommending that you “carefully try to capture the seeds before the wind blows them all away. By doing this and disposing of the seeds, the problem will eventually go away in two years.”

Meanwhile, Gary Grimes of Blue Point reports he’s “had this thing come back every year” and has no idea what it is. Senesac isn’t 100 percent certain, either, because to properly identify it, one would need to see it while blooming. That wasn’t possible, because when I reached out to Grimes, he’d already yanked it out.

Still, Senesac’s best guess is “it looks like a large vegetative stage of spotted knapweed” or a relative. Knapweed is an invasive weed with the potential to cause great damage, so Grimes did well to remove it. When allowed to flower, each plant produces up to 300 seed heads, each containing 30 to 40 seeds, which are easily carried by wind and water. In addition, the roots of spotted knapweed exude a toxin that can kill plants and trees growing nearby. If you spot this miscreant in your garden, pull it up and let it sit in the sun, preferably in a sealed plastic bag, until it wilts and shrivels, ensuring it’s dead, then put it in the trash. If you have too much of it to remove manually, mow over it repeatedly as it grows. This will prevent it from going to seed and eventually starve its roots.

Over in Centereach, Robert DelPrete has found a low-lying ground cover sprouting and spreading under his cherry tree. “Is it a weed or a flower?” he asks.

“This is definitely a sedum,” Senesac said, adding he’s “pretty sure it is called Sedum sarmentosum or stringy sedum. Also known as stonecrop, it probably was planted under DelPrete’s tree, perhaps by a previous owner. The tough-as-nails perennial ground cover is valued for its quick spread, filling in bare patches under trees and growing over — and in crevices — in rock walls. Too much of a good thing can turn invasive, however, so it should be avoided in areas where it can choke out and kill tender or slow-growing plants.

Theresa Nastasia of Stewart Manor reports that she cut down her lilac last year and now is seeing an unusual plant in the area. “I thought it was a weed until I noticed this flowerlike growth. Should I pull it out?”

This new visitor is “either Commelina communis L. or another species of dayflower,” according to Senesac. It’s an invasive plant from Asia that’s become a weed in many parts of the United States. Pulling by hand can effectively eradicate it if you are careful to remove all its roots. This is most easily accomplished when the soil is moist.

Be aware, however,  that often the stems will snap as you attempt to pull it. Broken stems, if left behind, can root into new plants, so be sure to dispose of them in the trash.

Kathleen Ledford found a plant “peeking out from under the hosta” in her Yaphank garden one day, and “two days later, it was a few feet long and twining around my porch railing!” Ledford, who observed “it seems to grow at an amazing rate,” said its stems snap when she attempts hand-pulling. The weed, she says, has a “hard, nutlike root about the size of a cherry tomato.” Although she’s been cutting it back regularly, Ledford said she’s waging a “losing battle.”

Senesac was especially interested in this weed, the native wild yam (Dioscorea villosa) as coincidentally, he’s studying it and its invasive look-alike, the Chinese yam (Dioscorea batatas). Native to North America, the wild yam has been used as a medicinal herb for centuries. A hormone from the plant, diosgenin, was used to develop the first birth control pill and is now marketed as a supplement to alleviate menopause symptoms. Although valued by some as a quick-growing vining plant, it can be eradicated by digging out its entire root system. Important: Because some look-alikes are toxic, avoid self-identification for the purposes of ingestion or topical application.