So my garden has quite a lot of bindweed. I am planning to build a keyhole garden. Will a few inches of cardboard at the bottom be strong enough to stop the dreaded weed invading? I have some of that plastic weed suppressing roll, would it be ok to put some of that under the keyhole to be sure or does the keyhole need to have access to the soil beneath?
There is some controversy about whether cardboard is too toxic to bring into the food garden. If you have a tall enough garden and thriving soil food web toxicity is not a big concern of mine. I would never put plastic in my garden. You are right that the garden needs to have contact with the earth to thrive.
I've been observing bindweed for about five years now and I've learned a lot. My preferred tactic is to wind up the stems and cover with a rock or brick. If you can go around every couple of days and tuck any new sprouts under the rock it will decrease the vigor of the roots. In my experience, trying to get all the roots out is only sort of effective, as the roots snap easily, the plant thrives in disturbed soil and every time you break the root it secretes a hormone that makes it grow faster.
I've had success with beds deeply mulched with straw on the beds and wood chips over cardboard in the paths. You don't even really need a rock, you can just tie the bindweed up and tuck it under the mulch
Ah, I will try that rock trick with the bindweed in the rest of the garden.
Cardboard is toxic??? The keyhole garden is half built now and has 4 layers of cardboard at the bottom! This is very concerning. How tall is tall enough for the keyhole garden not to be affected by any toxicity?
I still have time to remove the cardboard if necessary. I don't want to have doubts when I am eating my lettuce!
If you have cardboard now, don't sweat it. Do some research and see if you want to continue the practice in the future.
The great cardboard debate rages on...
The main arguments are
A) Cardboard is toxic. Don't use it.
B) Cardboard toxicity is not a problem because microbial activity renders it inert.
If you are scared of the toxicity and want it out, get it out.
As for bindweed, I have found that if you disturb the soil and are a little vigilant in the following year you can get it down to maintainable, acceptable levels.
Bindweed is like crabgrass-bermuda grass, you either wage an all-out and finally losing battle with it or you find a way to live with it. Here it only shows up from June to August, so we just weed more at that time. Bindweed and cinquefoil are our two main weeds.
In other news, bindweed has long roots that dynamically accumulate nutrients to the surface where your plants can use them. Another reason to keep them in check but not eliminate them.