WHAT YOU'LL NEED

1. Soil

• Lightweight soil. Due to the weight of soil, especially wet soil, it is best to use lighter soils such as perlite, vermiculite, peat moss, coconut husk fibre and rockwool pad (a Danish product that can completely replace soil without harming the plant)—these add bulk without jeopardising the stability of the plants.

• Fertilisers. If you use these types of soil, they need to be mixed with fertilisers, preferably organic ones, to ensure the survival of your plants.

• Minimum soil. Many plants do not need 12 inches of soil for healthy growth so use a minimum amount of soil as possible. Select plants that require less soil depth such as ones with shallow and spreading roots.

2. Water

• Water access. Water on the roof is ideal—i.e. a water tap.

• Water pressure. Moderate to high water pressure is preferable. If the water pressure is low, there may not be enough force to have a long drip system or soaker hose. In this case, several shorter hoses on separate zones would get water to all the plants.

• Drip irrigation system. This is an effective way to get a steady amount of moisture to your plants and can be put on a timer for efficiency and lower maintenance.

3. Plants

• Hardy or indigenous plants. Use plants of these varieties, as they are more capable of withstanding the harsh climate conditions of a rooftop such as sun, wind, rain fall, soil depth, shade and air pollution.

• Roots. Choose plants that have shallow root systems rather than ones that require higher soil depths making the container too heavy to be sustainable on a rooftop.

• Growth conditions. Consider where your plants have been grown prior to being planted on the roof and try and get plants that have been subjected to similar conditions.

• Plants and garden types. For an inaccessible/extensive garden, mixtures of grasses, mosses, sedums, sempervivums, festucas, and irises—plants native to drylands, tundra and alpine slopes are best suited for this rooftop environment. Accessible/intensive gardens can support just about any type of plant, provided that special protective precautions are taken for more sensitive plants such as using windbreaks and shading.

• Windbreak plants. Choose plants that have thick leaves with hairy or waxy surfaces as they tend to be stronger and lose less water to evaporation.

4. Containers

• Appropriate containers. Although terra cotta pots look good, they are too porous to conserve water under rooftop conditions. Plastic pots do a much better job.

• Size. Do not use anything smaller than 20 cm (8 inches) in diameter because there isn’t enough soil mass relative to the exposed surface area in a small container to hold much moisture for very long.

• Alternatives. Plastic pails that restaurant food supplies come in (usually 3.5 gallons or 16L) are perfect for a single tomato, pepper or eggplant. Poke a few holes in the bottom, add an inch of gravel for drainage, covered by a filter cloth to prevent the sold from coming out the bottom. Fill it about 2/3 full with a light weight soil mix, leaving room for much, and you have a perfect rooftop garden plant container.

• Larger planting beds. You can build wooden planter boxes or adapt packing crates. To do this, line with plastic, cutting a few holes in the bottom for drainage, and also fit with insulation on the inside, especially if you are planting anything that will be over wintering on the roof, such as perennials, shrubs or trees. These plants can be damaged by repeated with thawing and re-freezing in the winter and the interior insulation helps to minimise the thawing of the soil.

• Soil. No matter what kind of container you use, do not fill it to the top with soil. The lover soil level will enable the plant to get some wind protection from the container itself, as well as leaving enough room for a generous layer of mulch (at least 1 inch).