• Begin by consulting your municipal building inspector and checking by-laws.
• For safety's sake, you'll need to surround your garden with sturdy walls, which you can top with lattice to soften the wind.
• Keep containers lightweight by using soilless potting mixes, then stir in some moisture-retaining gel grains. After planting, mulch with small pebbles, shells or bark to lessen evaporation.
• Exposed to the elements, rooftop gardens experience strong winds that dry soil and snap spindly stalks and branches, so experts often recommend compact, bushy perennials and small shrubs. You could start with boxwood, cedar, eunonymous, ferns, grasses, lavender, hardy mini-roses, succulents, yews and slow-growing dwarf conifers, in containers from 50 to 60 centimetres deep. Then add some flowering annuals, perennials and herbs, in smaller pots. And include some native flowering and fruiting plants for the birds and bees.
• Place your planters on feet or bases that prevent water from pooling underneath or – even better – on wheeled platforms.
• Group pots together (for visual impact), and secure them so they won't blow away.
• Feed your plants. Monica stirs manure into the soil each year, and steeps compost tea bags in her watering cans.
• Monica empties all the clay pots and brings them indoors for the winter (so they don't freeze and crack), then cuts back the plants that stay outside and wraps them with burlap for insulation (bubble wrap is also great for this).
Tip-top info online
With roots in Montreal and branches that extend around the world, The Rooftop Garden Project develops and shares techniques for soilless container gardening, sells ready-to-grow lettuce kits, cultivates a demonstration garden (Jardin du Roulant) at McGill University in Montreal, and participates in projects in Cuba, Mexico, Morocco and Senegal.
Not only can rooftop container gardens and integrated green roofs help provide food security for people and wildlife, they also boost biodiversity and energy conservation. What's more, they reduce summer smog and heat, and help stop watershed contamination from rainwater runoff. According to Canadian Geographic, Toronto alone has 5,000 hectares of roof space it could retrofit; in the city there are already about 60 green roofs, with several more in the works.
Gardeners, students and teachers! Join The Great Sunflower Project and get a free packet of seeds to grow for your local pollinators.
Kids! Check out Science Buddies to learn how rooftop gardens help cities keep their cool – and how to make a mini rooftop garden that proves it.
Cooks! Find instructions to grow a one-pot herb garden for a rooftop, patio or balcony.
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