Controlling Thistles

What to do when thistle overtakes Juniper beds

By: Sandy Feather 2009
Penn State Extension

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. Is that true? Will this kill the junipers, too?

A. Vinegar is registered as a non-selective, 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. 

Thistle Seed Stockpile

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. It is non-selective, so vinegar could damage the junipers if you drip it on them while treating the thistle, but it would not kill them outright since it is not systemic.

Worst Thistle of All?

There are a number of species of thistle that are troublesome weeds, but Canada thistle is by far the worst. Most species of thistle are biennial, which means that they grow from seed and form a basal rosette of leaves the first year, then bloom and set seed the following year. Canada thistle 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. 

Thistles in a bed of spreading Junipers
Thistles took over this Juniper bed

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. You still will need to make repeat applications to get rid of the thistle infestation once and for all because thistle seeds present in the soil will continue to germinate in future years. Depending on weather conditions and weed species, glyphosate takes ten to fourteen days to kill weeds completely. 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. Glyphosate does not have any soil residual activity. You also have to be careful not to get glyphosate on your junipers because it will kill or damage any green plant tissue it contacts.


Herbicide Application Approaches

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 any 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 of the thistle. That way you coat as much surface area of the thistle with the herbicide as possible, yet avoid getting it on the junipers. You may still have to hand pull any thistles that are growing very tightly with the junipers. For example, if you have to pull a stem of juniper out of the way of thistle to apply the herbicide, and the juniper would touch the treated thistle after you let go of it, you should hand pull the thistle rather than using glyphosate.

Thistle problem enhanced by other weeds

There is a tool called a wick, which is similar to having a sponge on a broomstick. Using a wick may make it a little easier to reach the thistle. However, you may feel you have greater control over a sponge in your hand. It is important that you do not get glyphosate on the junipers. If you do, immediately prune that piece of juniper off and dispose of it to prevent the glyphosate from being absorbed and damaging the entire plant.


Photos of Spreading Junipers

Controlling weeds

Sprayer safety tips



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