Urban Rooftop Gardens

Large roof terrace with horizontally trained plane trees (Platanus) and wooden dining furniture, October. Part of a series, image 4 of 21
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Rooftop gardens can be an oasis in an otherwise built-up urban setting. Gardening on a rooftop has a number of benefits—including privacy, no deer, and good sun—but there are several things to consider about urban gardening before you start planting. If you have decided to create your own rooftop garden, here's how to get started.

What You'll Need

  • Tools
  • Containers
  • Soil
  • Fertilizer
  • Water
  • Plants

Gathering the Tools

You will need far fewer tools for a rooftop container garden than you would for a traditional ground-level garden. You'll be doing a lot of scooping and filling, so a trowel and a soil scoop are the first tools you'll need. A small tarp will come in handy when you are emptying soil, to keep from making a mess.

The only other essential tool would be a good pair of pruners to prune and clean plants. After that, the rest of the tools you'll need all depends on what you're growing and how much maintenance you intend to do.

Choosing the Containers

Containers are where you can express your style in a rooftop garden, but that's not to say you need to spend a fortune on them—although you easily could. Besides aesthetics, there are two things to keep in mind when choosing your rooftop containers are size: weight and material.

You will need containers large enough for the roots of whatever plants you choose, but the weight of the container becomes an issue if you are worried about how much your rooftop can support. Remember, containers get even heavier when you water the plants. Traditional materials such as clay, terra cotta, and cement can be quite heavy.

Plastic pots and the newer synthetic containers are light enough to lift, but you also need to consider balancing the height of your plants. A tall or top-heavy plant, like a small tree or a tomato plant full of ripening fruits, will topple over in a lightweight pot. If your rooftop garden is windy, heavier containers are a must.

In addition to being heavier than synthetic materials, natural materials such as clay, terra cotta, and ceramic also tend to heat up more than synthetics. When the pot heats up, the soil and roots heat up and the plant requires more frequent watering. You can help somewhat with a little shade and mulch the top of the containers, but drip irrigation or self-water pots will make your life much easier.

The Right Soil

Soil is often the least interesting thing to a new gardener, but it is the most important part of your garden. Good soil means healthy plants and less work for you. If you are growing in containers and raised beds, you will have the advantage of bringing in soil (rather than being stuck with what's already on the ground). The amount of soil needed will vary, so research your choices, before you plant them.

There are several good potting mixes on the market, but you can mix your own by combining three parts compost or composted manure with 1/4 part peat for lightness. Add a handful of perlite per pot, for improved drainage.

The soil in containers needs to be replaced periodically, usually every spring. You can lift and repot or simply top dress the existing soil.

Selecting a Fertilizer

Container plants, such as in an urban rooftop garden, will require regular fertilizer. Even a great potting mix will become depleted over time, as plants take up the nutrients and the water leaches them out. The larger a plant grows and the more water it takes, the faster the soil is depleted. The type of fertilizer you use will determine how often you will need to fertilizer, but every two to three weeks should be sufficient.

There are many good fertilizers on the market, including increasingly more organic choices. A water-soluble fertilizer is the fastest way to get the nutrients to your plants, either by watering the soil and getting it directly to the roots or by foliage feeding.

Picking Your Plants

You can plant virtually anything in a container, but a lot of plants are labeled as "great for containers"—this is a good place to start. Because most rooftop gardens get a lot of sun and are potentially very hot, during the day, drought-tolerant plants are recommended.

Take into account the rooftop's sun exposure and hardiness zone. Because rooftops hold heat, they can create microclimates that hover about a zone higher than a garden on the ground would be. You'll need plants that can handle the heat, but since they are in containers, they probably won't have better cold tolerance than ground planted gardens.

There is no list of certified rooftop hardy plants. Your choice of plants will require some trial and error. Honestly, with the exception of tall trees, there is little you can't grow on a rooftop. Many rooftops are little microclimates that can overwinter plants that would not be hardy at ground level. And if you have space and are willing, you can bring tender plants indoors for the winter. Annuals will require more frequent watering but will eat up the sun.

Some good rooftop plant choices would be plants with limited root systems, that don't need a lot of soil, such as herbs and vegetables and smaller to mid-sized perennials. Trees and shrubs require more soil and larger pots, but you need fewer of them to make an impact.

Rooftop Gardening Tips

  • Pot feet will lift your pots off the floor, allowing for better drainage.
  • Don't let cost keep you from gardening. A kiddy pool makes a wonderful raised bed, suitable for growing just about any kind of vegetable.
  • If you are going to be moving plants around or moving them indoors and out, place them on locking dollies.
  • Even a rooftop garden will be subject to pests and problems. Insects can fly, as can spores, so monitor your plants and try to catch problems while they're small.