Roof Gardens

If you have a peaked roof, roof gardens may not be not for you. But what about garden plants on your shed, chicken coop, or other flat-topped structure?

Roof garden of Rockefeller Center in Manhattan (Jwilly77) CC BY-SA 3.0

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We are not going to get into the details of how to install a roof garden here because that would mean talking about moisture barriers, structural integrity, and a bunch of other topics beyond my skill set or interest level. You can check out this article for more of that information. 

Instead of learning all the technical stuff needed to safely build a large-scale roof garden, we are simply going to explore roof gardens and learn a little about what they have to offer. Before we get started, we need to clarify the difference between green roofs and roof gardens. Roof gardens incorporate container plantings, seating areas, and outdoor living space, while green roofs are living blankets of plants installed primarily to improve insulation. Sod roofs are a type of green roof.

Traditional sod roofs (Erik Christensen) CC BY-SA 3.0

Roof gardens as temperature control

It has long been known that a layer of soil and living plants on a roof helps stabilize temperatures. Sod roofs have been used this way for centuries. Modern science now backs these traditions up with measurable differences. According to researchers N. H. Wong, et al, “the plants layer can shield off as much as 87% of solar radiation while a bare roof receives 100% direct exposure.” Imagine reducing your air conditioning costs by 87% each summer.

​If your shed or chicken coop get too hot in summer, you just might want to consider putting plants to work. What about building a green roof cover for your driveway? You could significantly reduce the heat island effect experienced by nearby plants, keep your car cooler, and it could look amazing!

Temperature profiles of rooftop gardens (above) vs reference roof (below) at different times of day (Karen Liu) CC0

Salad garden on roof of Palais de Tokyo, Paris (Jean-Pierre Dalbéra) CC BY 2.0

Roof gardens and rainwater

In cities around the world, rain falls on buildings and concrete, collecting car exhaust, trash, dust, grime, and who knows what else. This polluted water is then carried to our lakes, steams, oceans, and aquifers. Not good. Roof gardens reduce that run-off by absorbing the water and using it to provide for plants.

Paris roof garden (cocoparisienne) Pixabay

Roof gardens as habitat

Let’s face it - city dwellers rarely have access to enough natural surroundings. Roof gardens can offset that lack. Wildlife benefits in similar ways. Roof gardens provide habitat for a wide variety of native birds, animals, and beneficial insects. A series of roof gardens can also create a corridor for migratory birds and insects

Monaco roof gardens (Elvira Groot) Pixabay

Basic roof garden design

If you are sold on the roof garden idea and want to move forward, here are things you need to consider:

1. Is your roof storing enough? Ask an expert.
2. Find a water source.
3. Protect your roof with a moisture barrier.
4. Decide on containers and/or raised beds.
5. Add potting soil. [Note, peat is often added to roof garden potting soil to reduce weight. Because harvesting peat is so destructive to the environment, I do suggest its use. Add vermiculite instead.]
6. Select plants.

Rooftop garden plant selection

Rooting depth is particularly important when gardening on a roof. Check this list of plants and their minimum rooting depths to help you select the right plants for your roof garden:

• Less than 12” - basil, chives, cilantro, onions, oregano, parsley, radish, shallots, spinach, summer savory, tarragon, thyme
• 12” - arugula, bok choy, cabbage, fennel, garlic, Jerusalem artichoke, kale, kohlrabi, mint, rosemary, squash, strawberries
• 18” - beets, Brussels sprouts, carrots, cauliflower, celery, chiles, collards, corn, leeks, lettuce, mustard greens, Napa cabbage, potatoes, tomatoes, turnips
• 24” - beans, rutabaga, sage
• 36” - chard, cucumber, eggplant, fava beans, peas, peppers, rhubarb

Plants that can withstand a lot of wind and sun are also good choices. Succulents and most herbs certainly qualify. Remember, installing a roof garden can reduce summer energy costs by 25% to 80%.

Plus you get fresh herbs and vegetables!

Rooftop kitchen garden (Manfred Antranias Zimmer) Pixabay