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Sandy Feather: Ridding junipers of thistle requires targeted herbicide

Saturday, March 10, 2001

Q. We have an established bed of junipers that grow on a fairly steep hillside and help to support it. It is very important that these shrubs be maintained so that the hillside does not collapse. The problem is the thistle that grows among the junipers. If not controlled, the thistles grow so large and tightly packed that the junipers are crowded and start to die back. We have been hand-picking the thistle, which is a very unpleasant, tedious and difficult job because of the incline of the hillside. Is there some type of product that we could spray that would kill the thistle and prevent it from growing but would not harm the junipers?

A. Unfortunately, there is no herbicide that will selectively kill the thistle without harming the junipers. However, you can carefully "wick" glyphosate (Roundup) onto the thistle without getting it on the junipers. Roundup is a nonselective herbicide that will kill or severely damage any green plant tissue it contacts, so you must be very careful with it. It is systemic and will translocate down the roots of the thistle without your having to completely saturate the thistle with it.


Send questions to Sandy Feather by e-mail s or by regular mail c/o Penn State Cooperative Extension, 400 N. Lexington St., Pittsburgh 15208. Due to volume of mail, personal replies cannot be provided.


One way to do this is to mix the Roundup according to label directions in a small bucket rather than 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, then pull the sponge up to the top. That way you coat as much surface area of the thistle as possible yet avoid getting it on the junipers.

You may still have to handpull any that are growing very tightly with the junipers. If you have to pull the juniper away from the thistle to wick the Roundup on it, you should handpull that thistle instead.

There is a tool called a wick that looks like a sponge on a broomstick. Using it may make it a little easier to reach without killing yourself on the hillside. However, you may feel you have greater control over a sponge in your hand. It is important that you do not get the Roundup on the junipers. If you do, immediately prune that section off the juniper to prevent the Roundup from being absorbed and damaging the entire plant.

Whether you hand weed or use Roundup, try to get rid of the thistle before it goes to seed. It sounds as if this has been a problem for a while, so there is already a good bank of thistle seed in the soil. Those seeds will continue to germinate until the bank of thistle seed in the top inch or so of soil has been exhausted, which could take a number of years. Depending on the species of thistle you are dealing with, you may be able to control germinating thistle seeds with a pre-emergent herbicide such as Snapshot (isoxaben + trifluralin).

On a happier note, if you have grassy weeds growing through junipers, there are sprays that will selectively kill the grass without harming desirable plants. These selective herbicides include Ornamec and Fusilade II (fluazifop-P butyl). In junipers, these products should be applied as a directed spray. This means you should spot-treat the weeds without drenching the junipers. But if you get a little overspray on the junipers, it should not kill them.

They are also labeled to control grassy weeds in other groundcovers such as bugleweed (Ajuga reptans); cotoneaster (Cotoneaster spp.); English ivy (Hedera helix); pachysandra (Pachysandra terminalis); and creeping myrtle (Vinca minor).

There is another herbicide that selectively kills grassy weeds in broad-leaved groundcovers called Vantage (sethoxydim). It is not labeled for use on junipers, but is labeled for the other groundcovers I have listed.

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