Few gardens are more satisfying — and challenging — to create than rooftop gardens. It's an almost magical process, creating a bit of beautiful wilderness in a barren urban spot.
The most challenging part of creating a rooftop garden is dealing with the weight of soil and water. If you're just using a few small pots and portable planters, and a little patio furniture, you should be fine. But even a modest-sized rooftop garden's weight can cause problems — remember how those 1970s-era waterbeds made floors sag?
Consult a building inspector or construction engineer before doing any serious planning. You can hire them for an hourly fee, and the consultation will likely be brief. If your plan includes building any permanent or semi-permanent planters, trellises, railings and the like, you probably need a building or zoning permit. If you rent, you of course need your landlord's blessing.
When planning your rooftop garden, consider the following:
- Keep it safe. Install a railing that's waist high for most adults — city codes might specify the height. The railing should be closely spaced to prevent small children from falling through.
- As a rule, annuals and perennials need about 12 inches of soil to thrive. Small trees and shrubs need 18 inches. And taller trees need about 2 feet. Use a potting mix instead of garden soil (which is much heavier), not only for the roof's sake but because hauling soil up stairs and elevators is quite a job. Reduce how much potting mix you need by building planters with false bottoms and layering empty, capped 2-liter soda bottles with lids on at the bottom of large planters.
- Provide water. Plants dry out quickly in full sun and wind, and often require twice-daily watering in hot weather. If possible, have a plumber install an outdoor faucet on an exterior wall of the roof — again, you might need a permit. Otherwise, shop online to find hoses that attach to kitchen or bathroom sinks.
- Study how rain drains from the roof and arrange planters so water moves to drainage channels. Put pots in saucers. You might need to install flexible PVC pipe or other drainage systems at the bottom of large planters.
- Screen wind. On tall buildings, wind can make gardening difficult. If you have a windy site, build a screen of some sort that will filter prevailing winds, protecting plants and making it more comfortable for people. Consider pergolas, lath screens, wood panels and more.
- Add flooring. Roofs are seldom covered with people-friendly materials. Lay down a series of jute outdoor carpets, build a wood platform (like a mini-deck), or check out the interlocking high-tech materials made for garage floors. Just be sure they're also sunlight resistant.
Plenty of Plant Options
It's fun to choose plants for a rooftop garden. Go for a variety of plants. Evergreen and small trees are great in containers; if the site is very windy, you may need to run a wire from the tree to the building to prevent tipping. Annuals and perennials are ideal. And since most rooftop gardens have lots of wall space, be sure to take advantage of it with vines on trellises and hanging baskets.
In the northern third of the U.S., most plants in containers freeze solid, while plants in central third can be severely damaged by winter lows. You have a few options:
- Choose plants that tolerate a much colder climate than you live in — for example, in USDA zone 6, grow plants hardy to zone 4. Grow them in unbreakable containers so the pots don't split during freeze/thaw cycles and insulate the containers in winter.
- Treat your rooftop plants as annuals and replace them each year.
- Move plants indoors for the winter.
Special Touches
A rooftop garden can offer more than just plants. Consider a water feature. A small plug-in fountain or a tiny pond adds life, sound, and movement to a rooftop garden. Keep it small and it will contribute minimal weight and maximum pleasure. Just make sure to replenish evaporated water.
Outdoor lighting adds hours of enjoyment to your rooftop garden. Low-voltage systems made for gardens can be tricky to adapt to roof gardens, but check out what's available and get creative. Or opt for solar lights. For higher voltage lighting, call on a certified electrician.
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