Gladiolus add lots of pizzazz to container gardens in sheltered spots.
Gardening on a rooftop or exposed balcony is a demanding endeavor. The challenge has given me a new appreciation for plants and their tenacity for life. My rooftop container garden has become both a source of pride and inspiration. But it took a lot of thought and planning to get it established. If you're starting from scratch, here are some things to keep in mind.
The first step is to design the space. What's the best location for the plants so they receive the right amount of sun and shelter? Where do you want the barbecue grill, the umbrella stand, the chairs and table? On a small balcony or patio the decisions are simple, but large spaces require more creative thinking.
A major obstacle is transporting the soil, pots, plants, and other materials. If you are lucky enough to have access to an elevator, it may not be so difficult to move large pots around. Those of us with only stairs are in for a struggle, but at least we get a good workout. Cleanliness is also an issue. Plants, bags of potting mix, shoes, and tools have a way of making a mess. I am often sweeping up dirt and leaves from the common stairs.
Consider placing containers to screen unsightly vents, AC units, cables, etc. Allow plenty of room for cooling units, dryer exhausts, and other pipes to vent properly. Place containers where they block or at least direct attention away from those utilities. Besides screening unappealing views, containers can enhance beautiful vistas. Lakefronts, shores, riverbanks, treetops, parks, and even attractive buildings make gorgeous backdrops.
Wind is a major factor on exposed balconies and rooftops. On my Chicago roof deck there is a constant breeze, and the gusts are much more powerful than those at ground level. Wind has the potential to desiccate plants as well as knock them down. A dry southwestern breeze can suck all the moisture out of a container in a few hours. Hanging baskets and railing planters may require watering several times a day during hot and windy days. Strong gusts of wind can break stems, stunt growth, and tear large leaves. Suitable plants respond by growing smaller, thinner leaves.
Asiatic lilies can live in large pots year-round if they are protected from extreme cold in winter.
A few years ago I planted okra in a rooftop container. The first leaves grew normally, but by July the lobed dissections became deeper and the blade width thinner to reduce wind resistance. The plants topped out at 3 feet, half the height of their community garden-grown brethren, and the leaves looked totally different. The harvest was only a fraction of normal. Although it survived, I would not call okra a successful rooftop plant.
Because of the wind, taller plants, in general, don't perform as well. Weak-stemmed plants (tomatoes and dahlias, for instance) need support. Without a solid structure or protection from wind, they can flop and break.
Heat is another factor to consider. Rooftops are typically hotter than other parts of a building. Urban roofs (blacktop and reflective) are notorious for their scorching temperatures. Containers literally bake from radiant and convective heat. Plants that need cool roots suffer the most. Temperate woodland and boreal plants are out of the question. I have lost heaths, andromeda, hellebores, Solomon's seal, jack-in-the-pulpit, and monkshood, among many others.
The stifling heat combined with the intense light creates desert-like conditions. Rooftops have high evapotranspiration (evaporation plus transpiration) rates. Full exposure to wind, heat, and sunlight, with the resulting rapid loss of water, limits plant selection to those that can take a beating and a baking. (This explains why most green roofs are simply patches of invincible sedum plants.) Tender plants and small containers require lots of attention (mostly watering) to survive.
Large containers work best. First, they are heavy enough to withstand wind gusts. The increased soil volume means more moisture-holding capacity, which means less water stress for the plants. Large containers also allow room for combining different types of plants. Bulbs, annuals, perennials, and even woodies can be combined for multiseason interest.
Dollies are indispensable for moving around large planters.
Organic potting mixes make the best growing media because they are lightweight, enriched, and loose. The organic particles act as sponges by absorbing excess water and nutrients. Roots pull water from the particles during dry times. Gardeners can make their own organic mix, but it is easier to buy a bag of soilless potting mix. The mixes are already sterilized, plus they come in handy sizes.
Finally, the plants you choose should be resilient. The largest container with the best potting mix will fail if the wrong plant is chosen. Consider your climate. If you are in an extremely arid environment, choose desert plants. If you are on the coast and receive sea spray, pick salt-tolerant plants. If you are in the far north and expect early frosts, select cold-hardy plants.
Although the conditions can be extreme, some plants relish the sun and exposure. Short-grass prairie plants and desert and Mediterranean natives are particularly happy on a rooftop. Herbs like sage, thyme, hyssop, and rosemary require little attention. Sedums, hens and chicks, agaves, and other succulents can thrive with no attention. Many of my best performers are fairly common, like serviceberry, mini roses, and Asiatic lilies. But with a little research it's easy to find less common gems, such as Aztec lily (Sprekelia spp.), yellow ice plant (Delosperma nubigenum), and Corsican hellebore (Helleborus argutifolis). Trying new plants is part of the thrill. When one unexpectedly succeeds, like my hardy altai lily (Ixiolirion pallasii), it's exciting and gratifying.
The challenges are many but the rewards outweigh them. We can go upstairs and enjoy sitting in a green oasis surrounded by brick, glass, and asphalt. Having fresh herbs so handy has been a boon to our culinary skills. Fresh cut flowers are only a few steps away. For us urbanites in condos and apartments, rooftop gardening gives us a taste of that old American dream: a backyard garden.