Invasive roots from surrounding big gum trees in Raised bed
I live in a hill area of rocks and big gum trees. Our house practically sits on a rock. To find a good spot of deep soil is hard. Whenever I find a spot to plant anything, initial growth are wonderful, but soon they die or turn unhealthy.
If I plant in a pot, I have to raise the pot up, so the root system from the big trees can't invade through the draining hole.
I am planning a raised bed veg garden. Since it is impossible to plant directly. I plan to cover the surface with the tree mulch, then cover with large black plastic sheet, then on top of it a large piece of corrugated iron sheet, Which I happen to have. I will build a raised surround with large limestone which I get for free. I also have plenty of mulch from recent tree cuttings. Can I put in the raised bed first layer with this tree mulch, sprinkle with chicken manure pellets, then layer with brown leaves, which are in abundance in my yard, then with manure and then layer with more brown leaves, then manure, until the desired height? Finish on top with top soil.
Will this raised bed work for my situation? Can I get some advice in improving it?
The last time I built a raised bed, I put a thick layer of plastic sheet at the base and built around it with bricks, and I put good potting mix for the veg bed. However after a year, the root systems from surrounding trees broke through the plastic and the raised bed were congested with roots from surrounding trees, choking my herbs and veggies.
I really want to try this and be successful. Please advice the best way. Thank you