I go by the depth required by local building department for foundation footers. The idea is they've already done the leg work in determining the deepest expected freeze depth. You can call your local building department or one in a nearby city. You want the depth for stem wall type footing, not slab. I'd expect something like 4', maybe a little less. That's for like the expected worst freeze for the next 100 years. Maybe that freeze depth will happen next year, or in 50 years, or never. So you could reduce that amount and just deal with losing plants some years. I think it's easier to remove the pot, remove the tuber from the soil and store in a 32-50F part of the house. Then replant in the spring. To me they just do a lot better when divided and repotted every year and fertilizer added. Moving the pots to a deep spot in the pond sounds easy but pots tip over, you need to remove the dead leaves and they don't do as well when not divided and fertilized. So I just think it's less work to remove completely.

Thank you for all your responses! You are right, it is a bit late - I should have asked this question sooner. The vegetable blend from Lyncso came pre-prepared (top soil, manure and redwood) and I don't really know what the ratio was. We just happened to have some of our backyard dirt (clay - san jose type, no idea what that means) in to fill the remainder - mixed well. So, it is more clay than initially intended. That said, I also think one of the comments was right on, I did "compact" the dirt a little as we put it into the bed (4x11x18) but it seems to keep sinking. Very easy to press on it and compact it. When I added this last bit of seeds for beans and broccoli rabe, I turned and compacted the soil a little by hand first to see if that will help. I'll definitely do Fava this winter in some of the bed, but I will also do other winter veggies. that brings up a question, in an area where continuous gardening is possible, when do you amend? I can imagine by the time the summer veggies are done I will already need to have added the broccoli, cabbage, spinach and other winters. Buckets sure seem easier :-)

Cocos nucifera has a very shallow root system. You could easily grow some of the smaller cultivars in 50cm of appropriate soil, as long as the soil extends sufficiently out from the tree. A problem with the concrete base may be drainage. The tree requires good sharp drainage, and will need a good foothold to withstand coastal tropical storms. If coconut production is not a requirement, there are many other palms which could work as well. Consider making the raised beds a bit deeper, by going further up with elevated planting areas.

g'day emily, doesn't sound right to me? either, reckon they would need to be well and truely rooted into the soil d to hold that top load of even foliage alone. why 8m tall?? reckon they look ugly at that height and fruit too hard to get at. my suggestion buy malay gold dwarfs and let them grow naturally they will look better in the landscape. also duty of care to consider should fruit fall from that height onto someones head. maybe the landscaper has a couple of trees he wants to off load so your friend is his obvious choice? still be thinking of punching a hole through the concrete with a jack hammer before planting over the top with larger growing trees. len Here is a link that might be useful: len's garden page