Q. I have carpet junipers on my hillside, and every year I wind up with thistles growing through them. I have been told that a solution of water and vinegar will kill the thistles. Will this kill the junipers, too?

A. Vinegar is registered as a nonselective, contact herbicide. This is not the vinegar you use in the kitchen; it is a much stronger formulation, up to 20 percent.

It works as a contact or burn-down herbicide, which means that it only kills the portion of the plant it contacts. The thistle will re-grow from the roots, and you will have to make repeated applications until you exhaust the carbohydrate reserves in the roots. There is probably a lot of seed present in the soil, and new thistle plants will sprout that you will have to treat in future years. Vinegar could damage the junipers if you drip it on them, but it would not kill them outright because it is not systemic.

Canada thistle is by far the most troublesome weed in the family. It is a rhizomatous perennial that spreads by its creeping roots as well as by setting prolific quantities of seed every year. If you leave a piece of rhizome behind when you weed by hand, it will grow and produce another thistle plant.

You may have better results with a systemic herbicide such as Round Up (glyphosate) because it is taken down to the roots and will kill the entire weed. Depending on weather conditions and weed species, glyphosate takes 10 to 14 days to kill weeds completely. You still will need to make repeat applications because thistle seeds present in the soil will continue to germinate in future years.

It is best not to till or disturb the weeds until they turn completely yellow or brown; otherwise, the herbicide might not have a chance to kill the roots completely. You have to be careful not to get glyphosate on junipers because it will kill or damage any green plant tissue it contacts.

One way to do this is to mix the glyphosate according to label directions in a small bucket rather than in a sprayer. Wearing chemical-proof gloves, take a sponge and wet it with the diluted herbicide. Wring it out pretty thoroughly into the bucket so that you do not drip onto the junipers. Wrap the sponge around the stem of the thistle, as low on it as you can get without touching the junipers. Pull the sponge up to the top, coating as much of the plant as possible. You may still have to hand pull any thistles that are growing very tightly with the junipers.

There is a tool called a wick, which is similar to having a sponge on a broomstick. Using it may make it a little easier to reach the thistle. However, it is important that you do not get glyphosate on the junipers. If you do, immediately prune that piece off to prevent the glyphosate from being absorbed and damaging the entire plant.