Answer: One of your most important goals should be to protect your plants against the elements. Wind is the main enemy of an elevated garden. Summer storms can cause lasting damage by breaking limbs, drying out soil and knocking over pots, while icy winter gales can freeze and desiccate leaves.
Be aware of where the prevailing wind usually comes from ? off a river or through a wind tunnel. Create a barrier on that side with a hedge or a more informal windbreak, preferably evergreens like hollies, which will ground the garden aerie visually and make it more inviting year-round.
Next, select a few shrubs with a wide range of seasonal interest to serve as anchors. You may love lilacs, but they offer a few days of glory followed by months of flowerless monotony. Instead, consider a white-flowering Hydrangea paniculata, like Tardiva, which blooms in high summer but keeps its flowers until frost, aging from white to pink-russet. Or pick a late bloomer that barrels through the heat, like the chaste tree (Vitex agnus-castus) with its spires of purple flowers, or summersweet (Clethra alnifolia), which has fragrant white clusters.
Choose plants that fit the way you live. If you work all day, for example, select ones with light-colored foliage that capture the ambient twilight, like lamb?s ears or artemisia, or focus on night bloomers, like tuberoses, Oriental lilies or moonflowers, whose heady scent will sneak through an open door on a warm evening.
If you like to cook, grow small pots of herbs or one large pot with a cherry tomato plant on a wooden support.
If you travel or go away on the weekends, pick tough species or succulents that can take some drought.
Once these choices are made, fill in the gaps with whatever plants suit you ? spring-blooming bulbs like tulips, dainty annuals like sweetpeas or alyssum for early summer and asters or tall subtropical salvias for fall. Think of them as the garden?s special guest stars that give a big show and then are replaced as they start looking tired.
Many city gardeners become discouraged because they put a plant outside and watch it fail. But as a terrace or rooftop becomes greener and more crowded, the community of potted individuals protects itself, providing a moist, humid environment for weathering the ravages of New York in July and August. The lesson: the more plants on your terrace (within your building?s weight restrictions, of course), the happier they ? and you ? will be.
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