Q. Would it be possible to garden in a raised bed on my driveway? (It's the only place that gets enough sun.) Or would it be too hot there for the plants?
- ---Penny; in the My. Airy section of Philadelphia
A. Ha! That's where some of the best sun on my property falls! In fact, I suspect many people's best 'garden sun' falls on their house, driveway, or some other place that's inconvenient-to-impossible to grow tomatoes!
Unfortunately, I can't use that sunny space at my place for my own growing. The only flat part of our driveway is right next to the road; way too close to traffic. And the 'driveway' that runs from there to our backyard slopes eight feet from the front of the house to the back! (Our front door takes you into the living room; but the back door goes into the basement.) It's a pitch better suited to a log flume ride than a garden.
But it sounds like Penny has a conventional flat blacktop driveway, which is too inviting not to give this idea a try.
Now: blacktop will absorb heat from that good sun exposure during the day; until, as she fears, it might generate enough BTUs to cook her plants, especially at the height of the season.
I would suggest she paint her driveway white—but can you even paint blacktop? So maybe consider whitewashing a section and see how well that temporary coloring holds up. My second thought is to get the Surround clay spray I use to protect my peaches and see how long that lasts on the blacktop; it keeps my peach trees a light-grayish white for a solid month. Either way, you wouldn't need perfection; just something to cut the heat absorption.
And she should also be able to find big rolls of white paper at someplace like a restaurant supply store; she could line the area between the beds with that. You know—the kind of big paper they cover the tables with at some 'family restaurants' so kids can color madly with crayons while the parents try to eat in relative peace. (Ask your local kid-friendly restaurant for the name of their 'tablecloth' supplier.)
Once you find a way to block out the black, frame out boxes that accommodate the size of the driveway but are no wider than four feet each—because just like with conventional raised beds, you want to be able to reach the centers of the boxes without stepping into their nice loose soil.
But I would not leave the bottoms open to asphalt. As with beds on soil (to prevent weeds or lawn popping up), I would cover the area underneath with a single sheet of thick cardboard to prevent contact with the 'tar'. And if she can locate a big source of white cardboard, she could make nice white islands with the boxes in the center; that would break the heat around the boxes and prevent contact with the driveway surface. (And hold up better than paper.) If the boxes end up sitting really flush to the 'ground', drainage might be an issue when we get a lot of rain, so I would suggest carving some small drainage holes in the bottom sides of the boxes—about half a mousehole each, lined with hardware cloth to keep the 'soil' inside.
Why 'soil' in quotes? Because with yours truly, the 'soil' used to fill containers (which these boxes will be more like than 'beds') ideally contains no garden dirt. The 'soil' in my raised beds and containers is a clean start. For this purpose and usage, the minimum mixture I'd recommend would be a 50/50 mix of rich black screened topsoil and high-quality compost. NO crappy regular garden soil; or composted manure; or whatever the heck is in those weird heavy bags labeled 'garden soil'. (What is up with them?)
Anyway, that mixture is the minimum. Ideally you want to lighten it up, as well as start with a nice rich clean blend that doesn't have your landscape's ratty clay soil or weed seeds in it. One way to achieve this lightness would be to mix in a huge bag of perlite or two….
Yes, gentle readers/listeners; I have become a perlite fiend. I use it everywhere. I even add perlite to my bags of seed-starting mix--which already have perlite in them. Hey—it's an organic mined mineral; and nothing is better at improving drainage and bringing air to the roots of plants. (And it's not a 'problem'; I can stop whenever I want to. Or, perhaps more honestly, when the factories that process perlite run out of volcanic glass to pop…)
Anyway, the best option would probably be 1/3 compost, 1/3 screened topsoil and 1/3 bagged soil-free mix that doesn't contain chemical fertilizers or those stupid water holding crystals. And a big bag of perlite. (Such a mixture is also very close in spirit to "Mel's Mix"; the combination that Square Foot Gardening guru Mel Bartholomew recommends for his famous system of intensive grid growing.)
Now, if Penny wants to use the driveway over the winter, I would switch to a table top gardening system that can be dissembled and stored in the off-season: sturdy outdoor tables on which she can arrange rectangular planters. She'd be able to grow herbs, greens, bush beans, peppers, eggplant—everything except really big tomatoes. For them, I'd do big round containers on wheels so they could be moved easily at the end of the season. (See our previous Questions of the Week on table-top gardening and growing tomatoes in containers for details.)
But if she doesn't need to use this section of the driveway, I would urge her to use a removable whitener, like paper or cardboard, so she can turn the heat projecting power of the blacktop back on when the weather cools and utilize that ambient heat to grow salad greens and other cool-weather crops in the Spring and Fall. (The blacktop would absorb heat during bright sunny days, no matter the air temp, and then radiate that heat back up for a few hours after dark.)
And if she wants to go for the Garden Gold, she should construct the boxes so that they're the right size to utilize hooped row covers—that physical protection plus the reflective black heat around the beds might just allow her to grow cold hardy greens well into the winter.