My raised garden bed is very shallow, only about 10 inches deep. Do you think it would be possible to plant a lilac tree in there? Maybe a dwarf? I have one in a pot right now that gives me a limited amount of flowers every spring. I would love to transplant it in my garden.
Unless drainage is poor or bed is infested with armillaria there is probably little reason to think it would not grow there. Lilacs are very commonly seen in this region.
What's underneath the raised bed? Lilacs grow like weeds around here on very shallow soils over limestone. I imagine there is some root pentration into limestone cracks but the majority of the roots exist in 12" of unirrigated soil.
Thank you for your answers. Underneath the raised bed is concrete; the garden bed is on a deck above the underground parking. I have heard that lilac roots are a tad invasive. Maybe not so great an idea?
Not terribly invasive compared to maples, willows, poplars. They'll only grow into areas that offer favourable conditions (water & nutrients), neither of which is prevelant in a parking garage. Most root damage is caused by either blocking of pipes (not a problem in your case), or root diameter growth opening up cracks (lilac roots are small in comparison with the worst offenders).