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.