Canada thistle is a hearty invasive that can be difficult to kill — and how to get rid of ants in a houseplant

For The Baltimore Sun

This prickly plant has been coming up more and more in our junipers. We keep pulling it out, but it’s like whack-a-mole. What can we do to clean it out of the entire bed?

Canada thistle, like many invasive plants, looks so harmless when it first appears, but watch out! It has a huge root system of fat roots that store so much carbohydrate energy, it comes back over and over, even from a small bit of root.

Don’t try to dig it out — that’s impossible unless the plant is a little seedling. In some situations, cutting it back and smothering it can kill some of it. In your juniper bed, you could exhaust the root system by removing all stems, then pinching back new growth the instant you see it. This could take years.

You also can combine stem removal with another option, applying a systemic herbicide (glyphosate) to go down and kill the roots. You’ll have to paint it on carefully, to avoid getting it on desirable plants. Search ‘Canada thistle’ on the University of Maryland Extension’s Home and Garden Information Center website and be sure to read the links, too.

Never let it flower and produce seeds.

Ants seem to be thriving in my houseplant pot. How do I banish them using non-toxic means?

Sounds like you have an ant nest in the pot. Warming weather will make eradication easier. Take the pot outside, drench it with water several times in 24 hours (or drench with a 1-2 teaspoon dish soap/gallon mixture). Then keep the soil saturated for a few days by having it sit in a saucer of water. The ants should bail out. You could also simply repot with fresh potting soil.

University of Maryland Extension’s Home and Garden Information Center offers free gardening and pest information at Click “Ask Maryland’s Gardening Experts” to send questions and photos.

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