Extensive green roofs are not well suited to vegetables and herbs because the growing medium is very shallow. For example, the University of California at Berkley recommends a soil depth of 9 to 12 inches for shallow-rooted vegetables like beets, lettuce, onions and spinach. That’s about 5 to 8 inches more growing medium than a typical extensive roof has.
Many herbs will grow 2 feet high and taller making them susceptible to tipping over from the higher winds at rooftop when they are rooted in very shallow soils. Watering becomes a problem too when trying to adapt plants to shallower than normal growing medium. They will dry out very quickly, and in some cases will need water multiple times each day.
The best bet for growing vegetables and herbs on an extensive roof is to use containers judiciously spaced along the edge where there is support from the wall below. The containers can provide the root depth they need. There may still be care issues like watering and the effects of wind, so this is definitely not an ideal growing situation.
Intensive green roofs are a completely different story. The deeper growing medium accommodates almost any variety of vegetable and herb you might want to grow. When there is an irrigation system in place, the garden is really not that much different from one on the ground.
Use beets, lettuces, onions, radishes, spinach, carrots and swiss chard for growing mediums up to 12 inches deep. Broccoli, cabbage and cauliflower like soils between 12 and 14 inches deep. For growing mediums from 14 to 16 inches deep you can plant cherry tomatoes, eggplant, peas and peppers. Deeply rooted plants like beans, cucumbers, potatoes, summer squash and tomatoes like planting medium that is 16 to 18 inches deep. Keep in mind that for the larger plants you can double the yields by making the growing medium just 2 inches deeper.
Many herbs are perennials so they will come back year after year. You have to select the ones that do well in your climate. Check the United States Department of Agriculture Plant Hardiness Zone Map to find what zone you live in, then select plants that are hardy in that zone. Rooftop growing conditions can be harsher than on the ground, especially in the winter, so you might consider adding a 4-inch layer of mulch around the herbs.
If you already have an extensive green roof and you want to add vegetables and herbs to it without resorting to containers then some modifications will no doubt be required. In this case ask an architect or structural engineer to assess the current strength of the roof and advise you how you should reinforce it so it can support the additional loads of an intensive green roof. If you can get another 5 to 6 inches of growing medium you will have enough for some herbs and vegetables. These roofs are sometimes called semi-intensive.
While many people may want to grow vegetables and herbs on their green roof because they perceive the produce will be organic this may not be the case. It might be organic from some perspectives as in not having any pesticides or herbicides used on it, but from other perspectives it may not be considered truly organic. That’s because there are restrictions on the types of materials that can be used in the plant growing medium and some mediums may contain restricted materials. Other factors may also prevent these foods from being certifiably organic under the Organic Foods Production Act.
Image Credit: Flickr user Living in Monrovia under a CC License.