Join Date: Aug 2003
Location: Have I been here all this time?
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No, I am not growing asparagus in a box. But I hope you'll let me chime in...
I'm not familiar with mint and thistle rhizomes. Are they deep and long? Can you dig out and remove/screen the dirt that will sit under the box, maybe six or 8 inches or so? Replace with what you know is clean. Put your impermeable to roots material down and then then the box and then fill it? Then, in the very center of the box cut an x or hole in the fabric where the asparagus can dive. If you work hard to deplete the rhizomes of the plants surrounding the box they may not have a chance to migrate under the box all the way to the center, then back up to earth. That would be an amazing journey, though I'm sure there's some weed out there that is capable of it.
I share your pain. Ours is of the quackgrass variety. The guy who lived in the house before we bought it hauled in 40 bales of straw when he built it as a way to build soil, rather than sodding it. Commendable intent, but it spread quackgrass all over the backyard. We tried pulling it. Got worse. Then we tried solarizing it. Nice in theory but it didn't work out for us. I've recently tried straight vinegar and I did see damage, but know it's only surface damage. Our current approach is to whack the tops off down to the dirt, rake up the debris and spray all baby shoots with vinegar as soon as they come up until the rhizomes are depleted. Depletion: that's our latest plan.
We also really like working with raised beds but are fearful the little shoots will come up though the bottoms and we'll be growing quackgrass in our newly-purchased soil. That would throw me over the edge. The approach we decided on is to use the whack/vinegar/deplete method all around the boxes as long as necessary, and the boxes are getting lined underneath with very heavy duty cardboard which we were able to source from an appliance delivery place. I'm thinking of actually stapling them to the bottom to make sure they stay in place and make a fairly tight seal. Over time I'm hoping we win the quackgrass war, the cardboard breaks down, and we have wonderful soil layers in the boxes. I have an asparagus plant that was overtaken by the quackgrass and I tried to separate it but I'm actually just going to pitch it. I simply can't take a chance that there is some fraction of a rhizome in that huge root system.