Comfrey is one of the most helpful and prolific plants in my system. But over the years, I've planted a few plants that I now wish i hadn't planted. And as any of you who have comfrey can attest, you really can't dig it out. Its a ton of work and you've got to make sure you get every little root or it'll just come back --- but now you'll have 5 or more plants growing where you used to have one.
Over the years I tried to pull it out but it just kept coming back. It was a lot of work and a lot of soil disturbance. So this summer I experimented with getting rid of it and I think I've found a simple solution. It takes about 2 minutes and very little work.
1. Stick a shovel or fork under the plant and pull out the bulk of the root ball. You can use this to plant a dozen more comfrey plants elsewhere, you to bless your friends and neighbors who don't have any growing in their system. I suppose you could skip this step -- just cut back the leaves. But by yanking the bulk of the plant out, you are stressing the plant and forcing it to push up new leaves. Thus, the plant will use all of it's final energy in an effort to re-establish itself above the soil.
2. Get a large black plastic pot or black plastic mixing tub. I've got a black tub that I use for mixing small batches of concrete. They are cheap enough at Home Depot or Lowes, and are a heavy black plastic.
Of the two, the tub seems to work better, but anything that seals out the light would seem to work. I suppose a 5 gallon bucket would work as well.
Why black plastic? It's a heat sink. The summer sun beating on a black plastic pot would make it very hot underneath. (A parked car with the windows rolled up can get up to 150 degrees). A white 5 gallon pail might not be big enough to cover the remaining roots, and wouldn't generate as much passive solar heat underneath it as something dark colored.
3. With your hands or a little trowel, dig a bit of shallow trench around the base of the plant so that you can situate your pot or tub slightly below soil level. Place the pot or tub over the comfrey and nestle it firmly into the trench. Then backfill up against the exterior sides of your pot. It shouldn't allow any light to get to any emerging plant leaves.
4. Put a rock or a couple of bricks on top, so your pot doesn't get toppled.
5. Wait 4 months.
Deprived of sunlight and over-head water, and subjected to the hot summer sun, the three plants I've tried this on all seem to have died. No Round-Up or salt or anything chemical is being used. It doesn't take much space, so I have planted other plants (squash and watermelon) around the experiment, and they have done great.
I'll leave the pots on top for another couple of months, but this seems to have done the trick.
If I knew how to post pictures, I'd do so, but I would imagine that you can visualize it.
"The rule of no realm is mine. But all worthy things that are in peril as the world now stands, these are my care. And for my part, I shall not wholly fail in my task if anything that passes through this night can still grow fairer or bear fruit and flower again in days to come. For I too am a steward. Did you not know?" Gandolf