We built raised vegetable garden beds last spring that have become infested by a weed that is so sharp and durable that it has come through a doubled layer of weedcloth under the beds. I pulled all the weeds out and laid down on top standard weight cardboard with neswpapers over the edges to block out light then a layer of black much over the cardboard/newspaper. Now more weeds have pushed through the cardboard/mulch combo. The weeds are rhizomes and grow in clumps. When the weed comes up, the first part is very spikey and actually hurts when you poke your hand on it. How do I control/kill it without toxic chemicals?
Weeds Coming Through Weedcloth, Cardboard Underlay and Mulch
Certified GKH Gardening Expert
It sounds like you have canada thistle, which is a vicious weed. The best way to handle these organically is to literally exhaust them. Keep a sharp pair of shears handy near the garden and as soon as you see one of these start to sprout up, snip it off. It will regrow, and as soon as you see it, snip it off again. After a few times, the plant will have depleted itself and will die.
The idea is that the plant needs its leaves to produce energy, but it also needs energy to grow its leaves. By keeping the plant snipped to the ground, you are forcing it to use a large amount of energy without having a resupply. Eventually the stores of energy in the roots are used up and the plant just can't go on. What makes this such a PITA weed is that it can store alot of energy in its roots, which give it the time to push its way through almost anything.