Small trees for rooftop patio in St. Paul, mn
I've done some reading and know that a rooftop environment is a tough place to grow most things, but I just bought a St. Paul condo and really want a couple small trees on my 15' X 15' rooftop garden. I was hoping to find a sour dwarf cherry tree, and perhaps a Hawthorn. Someone told me I may have to consider treating them somewhat like bonsai plants every couple/three years to keep their roots from getting too large. I know the pot will need to be large, at least 3'X3'X3', and perhaps insulated. I'll have water available on the roof and would put in a drip system. The wall of the condo could provide some winter protection, but the rooftop faces west, NW and the sun will be on the far end of the rooftop. Any suggestions as to trees I should consider or avoid, ways to manage their survival, any books on the topic...anything, would be greatly appreciated! Thanks, Deb
Ramsey County Minnesota
Below is a good reference from the Brooklyn Botanical Garden (you may have already seen it) on rooftop gardens, including trees:
*Choose trees for one hardiness zone colder than you would for an in-ground garden.
*Choose trees with shallow root systems to alleviate the need for "bonsai treatment."
*Use a good container soil mix, lightweight soil is best.
*Evergreens such as junipers are good in drying winds.
*Dogwoods and dwarf Japanese maples are good container trees.
*River birch is suggested by some but it does need significant amounts of water so might not be the best choice even with your drip irrigation.
*A Hawthorn tree (zone 3), which you suggested, is on the rooftop garden at the Chicago City Hall (windy city) so could be a good choice.
* http://honeyberryusa.com/honeyberry-plants-2.html lists some dwarf cherries hardy to zone 2. You're in zone 4 so that would be good for your suggested dwarf sour cherry. Some of these are descendants of the MN bred North Star. They vary in height although all dwarf.
Insulated containers are a good idea.
Hope this helps a little. A 15'x15' rooftop garden in the city of St Paul! Wow!