I already have thistles growing in my yard and landscaping beds.


DEAR AMOR: Spring is here. I can tell because thistles are already growing in my garden and beds are landscaped. I feel like I’ve tried everything over the years to kill these things to no avail. I blasted them with weed killer, cut them, pulled them (what I found out later is not the best idea). I even cut the stems and injected weed killers into the stalk with a syringe and they’re still growing back. Is there any way to eradicate these things for good? – Steve

DEAR STEVE: You are not the only one being tortured by these harmful, unwanted weeds. I was also patient with my annual struggle with them.

Like you, I have also observed that new, strong shoots appear in the same place after spraying and killing these weeds. That tells me the plant is very much alive

and that I just killed the visible stain with myriad deeply ingrained drive reserves ready to take their place.

One thistle that is hated in many states is known as a Canada thistle. If we understand the Canadian thistle’s growing habit, we can ultimately exterminate this enemy.

Canadian thistles have built up an enormous underground root structure more than we thought. Above-ground patch formation also takes place when one is allowed to flower and develop seeds. Seeds scattered on the ground can become viable even after many years. It will take several years for a fully developed two year old plant to be eradicated.

Some have sworn by a homemade weed killer using vinegar / salt / dish soap, but it won’t be effective. Even powerful commercial weed killers only kill what shows up above the ground. Because of this, there is no one-time strategy.

In my experience, chemical applications intended to damage the roots only work to a certain (shallow) depth. It won’t completely destroy its lateral, deeply established roots. The weeds’ underground nutrient reserves allow them to easily recover after treatment.

According to a study by the Colorado State University Extension, one key to controlling Canadian thistle is to put strain on the plant and force it to use stored root nutrients. Canadian thistle can recover from almost any stress, including control attempts, due to the storage of root nutrients. The return of the affected land to a productive state only takes place over time. Success requires a solid management plan that is implemented over several years.

The Canadian thistle management plan requires chemical treatment in the spring and fall for an extended period (years) to be eliminated. Purdue Extension provided the following conditions necessary for good foliar herbicide performance:

1. Sufficient soil moisture from the surface of the soil deep into the subsoil.

2. Green leaves, not withered and generally free from extensive damage from insects, disease, drought, frost and dormancy.

3. The shoot height is at least 10 inches in early June and 8 inches in the fall.

4. Flowers not fully open.

5. The thistle has not recently (within two months) been disturbed by tillage.

6. Do not mow or cultivate for 10 days after application.

Amor Chamness Cook is a graduate of Brigham Young University-Idaho and a master gardener for Purdue University Extension. Send gardening questions for Dear Amor to Dearamor@yahoo.com.

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