Root mass and depth of growth will depend on the specie of plant. The flowers may have had shallow roots that did not extend through the bottom of the floating island.
With agriculture most plants fall into a 4-1 category. What you see for above ground growth you will have 4 times that in root underground growth.
Low maintenance with out any weeds is the wild flower mixes. Choose one that you could like to have as secondary creatures like butterfly's, humming birds, ect.
These are self seeding for the next year. They will grow so thick that you will not have to worry about weeds and the birds just love the seeds in the fall.
Wild flower mixes. Cheers Don.
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