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.
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.