Four-Stresses to Kill a Weed

It’s probably 25 years ago that I sat in class and heard the general rule “It takes four stresses to kill a weed.”  By that I figure it takes four stresses to kill any plant – and I’m looking at my little alfalfa seedlings with a lot of sympathy.  They have experienced moisture stress.  I would think they have experienced heat stress – but there are heat tolerant varieties of alfalfa, and I was selecting for salt and water tolerance.  My alfalfa seedlings may still have a chance – I mowed the field high, to add one more stress to the weed competition.  I figure being cut down is at least one stress, and the alfalfa seedlings are much shorter, so they weren’t cut.

As I see knapweed bursting into bloom, I grab hold and pull.  In my pale, high clay soil, so much moisture has been pulled that at least half the plants come up with 3 to 4 inches of root – the soil is so dry that it isn’t holding the roots firmly.  I think pulling them up by the roots adds at least one stressor, just like a little herbicide adds a stressor.  Folks at the county weed department have spent a good portion of the 21st century adding insects that feed on knapweed – adding one more stressor.  It’s getting late now, but a little herbicide might go a long way with the other stresses.

Knapweed, flowering

Ox-eye daisy is classified as a noxious weed.  Since it has shallow roots, it is easy to control . . . which means it is easy to stress.  It’s persistence strategy is lots and lots of seeds – but one of the easy controls is healthy grass stands.  If the grass is thick enough, the daisy can’t get much of a start.  Grass makes better use of the soil nutrients.  Laird Byers used to call it a “poverty weed” occurring most frequently where fertilizers weren’t used.  The fertilizer strengthened the grass, and a little 2,4-D stressed the daisy a little more.  Along with this year’s drought, and a bit of mowing, the poor Daisy is likely to have three or four stresses in hayfields before a mild herbicide application. 

Canada thistle – like the daisy – produces lots of seeds.  Like knapweed, it has deep roots.  Still, a bit of 2,4-D works – it causes the cells that carry water and nutrients to grow non-stop.  It’s been around since the forties, and is a fairly gentle herbicide – though it got a bad rap in Viet Nam where it was mixed with 2,4,5-T in a compound called agent orange.  The problem was the dioxin in the 2,4,5-T.  The herbicide keeps the roots from coming back the next year.  We also have bugs adding to the thistle’s stress.

Ask The Entomologist, Plants

Knapweed, my current enemy.

At this time of year, many hill-slopes have turned a sharp pink-purple color. Whether you’re in Glacier or driving along 93, you’ll see its flowers in the cuts alongside the road. Here in Trego proper, you can often find it in ditches, or there’s an abundant field of it downslope from the Trego Pub. Knapweed. It’s everywhere, and looking far healthier than anyone would like it to.

Spotted Knapweed: note the black markings on the green, just below the flower blossom – these are the “spots”.

Knapweed (Centaurea sp.) is a genus of invasive plant that plagues rangeland across western North America. Its seeds made landfall on the west coast, back in the early 1900s, possibly due to contaminated alfalfa seed. Now, over a century later, knapweeds flourish across America, from sea to shining sea, more successful than they were in their homeland of southern Russia.

Thanks to the absence of the specialist insect herbivores that didn’t journey with it to the New World, and its bad taste, few things eat knapweed. And even if you are able to get something to eat it, with taproots reaching up to four feet deep, Spotted Knapweed is able to resprout with ease. Most knapweeds are only palatable to cattle early in their growing phase, quickly outcompeting grasses in overgrazed areas. To emphasize how terrible it can be in rangeland, feeding on Russian Knapweed may even prove fatal to horses.

Several common native North American grasshoppers, notably the Red-Legged Grasshopper, feed on knapweed, but they weren’t effective at controlling it. Biocontrol began in the 1980s, with introduction of knapweed-specialist weevils, flies, and wasps. Over the past 40 years they’ve proven quite useful – if there are enough seed-head feeding insects, many fewer knapweed seeds are produced each year. It’s still not a quick fix – knapweed seeds can lay dormant in the soil for at least seven years.

The Knapweed Peacock Fly: introduced for biocontrol, its maggots destroy knapweed seedheads.

These introduced bugs have done wonderful things. Knapweed is much less a problem than it once was, but biocontrol alone isn’t enough. If you happen to have knapweed on your property, the best thing you can do is to start spraying herbicide. I’ve been having decent success eliminating it with Milestone (Aminopyralid).

As best I can tell, knapweed’s one virtue seems to be a high rate of nectar production – some folks enjoy making knapweed honey.

What are your thoughts on knapweed? Have you ever tried knap honey?