But there's another reason to point those plants north.
"Vertical is way cool," says Bruce Butterfield, research director for the National Gardening Association.
"Way cool" comes in many forms: arbors, trellises, lattice frames, stakes, tepees, and tuteurs or obelisks, along with some unorthodox methods and materials that resonate with the frugal, the artistic, and the diehard do-it-yourselfers.
In her research, Sweet, a landscape designer from Los Altos, Calif., saw flowers and vegetables planted in rain gutters suspended from arbors or mounted on walls; tomatoes growing out of the bottom of industrial buckets dangling from fences; and vines clambering up vintage headboards or weathered wooden ladders.
"You can get really creative," Sweet says. "It's fun."
Vertical plants and props also earn their keep as architectural statements and problem-solvers.
They offer what designers call "exclamation points" in otherwise flat landscapes. They add lushness and depth, and save space. They enliven gnarly stumps and dead trees, and hide unsightly sheds, air-conditioning units, utility poles, and fences.
They also can provide privacy.
Last year, Megan Jann, a lifelong South Philadelphian, moved from one rowhouse in Pennsport to another just blocks away. She plans to re-create the old house's privacy screen this summer at the new place, which shouldn't be hard. Jann's system is pretty simple.
To the top of her 41/2-foot-tall cinder-block walls in the backyard, she bolted generic wooden trellises measuring 2 feet by 6 feet, next to the barred sides of her son's outgrown crib. She placed them horizontally, rather than vertically, so the wall became 2 feet higher and each "trellis" covered a 6-foot-wide swath.
She hung potted plants from the trellises, so the vines hang down, and filled pots on the ground with brightly colored bougainvillea and cardinal climber, mandevilla vines and orange or pink climbing roses. They were guided skyward with screws and fishing line.
"You're still letting light and air through with the trellis, but you're covering an ugly cement-block wall - and you don't have to look into the neighbor's ugly yard," Jann says.
Valerie Cutler, a retired art teacher from Chestnut Hill, isn't worried about privacy. She's interested in keeping critters out of her three community-garden plots.
Gardening up, Cutler discovered, is an eye-pleasing way to cover the makeshift deer fencing that surrounds the seven-acre garden that straddles Upper Roxborough and Whitemarsh Township. And it allows her to fully enjoy her flowering vines.
Last summer, her passionflower shot up and over the fence, and 6 feet across it.
"That whole section of the fence in bloom is just gorgeous and the pollinators love it, especially the bees," says Cutler, who also grows peas, pole beans, tomatoes, and blackberries vertically, on trellises made of 8-foot metal fence posts woven with vegetable netting.
"You can see everything better when it grows up, and your garden looks very lush," she says, "like you're out in the middle of the country."
Vertical excitement at Longwood Gardens in Kennett Square is generated by a much-ballyhooed "green wall" and exquisitely shaped topiaries, options that senior gardener Adam Cressman thinks are lovely to look at but not especially practical for home gardeners.
Cressman, who's in charge of Longwood's three-quarter-acre vegetable garden and grows 40 tomato plants in one bed, recommends store-bought supports or DIY trellises and posts made of cedar or locust with bird or deer netting stapled to the wood to support the plants. He sets his posts 7 feet apart, with two tomato plants between them.
As the tomatoes grow - most are heirlooms - Cressman weaves them through double strings of twine stretched horizontally at 1-foot intervals up the posts, to about 8 feet. "As the plants grow, I keep putting in new rows of twine," he says.
The system is easy and inexpensive, and looks like "an espalier of tomatoes," he says. The wooden posts, visible despite a growing froth of greenery, add a rustic and surprisingly orderly touch.
This method works well with vining crops, such as beans, peas, and cucumbers, climbing or rambling roses, and "twiners" like clematis and hyacinth bean. Some vines, among them climbing hydrangea and English ivy, attach themselves to rough surfaces like brick or stucco, without supports.
Cressman, who typically begins putting new posts and trellises in the ground at the end of March, also loves vine-covered arbors and arches framed with rebar, the ribbed steel bars used to reinforce concrete, and woven with repurposed tree limbs, attached with gleaming copper wire.
"Copper looks so nice, especially as it ages," he says.
As for rebar, Butterfield, of the National Gardening Association, is an evangelist for the stuff. "It's cheap, it lasts forever, and it's just like Velcro for plants," he says of rebar's effect last summer on his scarlet runner beans.
"Like Jack and the beanstalk," he adds.
Nothing like the tale of magic beans in March to fuel a fantasy of summer to come - the perfect vertical garden.
Contact garden writer Virginia A. Smith at 215-854-5720 or firstname.lastname@example.org.