Healing Above Our Heads? Green Roof Medicinal Herbs
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Terra rooftop garden, Colorado State University, Spur Campus, Denver, CO. Photo: Brian Adams.
Introduction
It is incredible to witness the growing popularity of rooftop agriculture in North America. There are few things more inspiring than being a part of the harvest of thousands of pounds (kilograms) of produce from urban rooftops over the course of a growing season. The produce often goes on to feed underserved communities as well as the general public. What better use could there be for an empty rooftop than to contribute to the resilience of a local food system?
Rooftop farms can produce the same types of vegetables as any other urban farm: leafy greens, tomatoes, peppers, and squash. We produce these vegetables and more on our rooftop farm at the Colorado State University (CSU) Spur Campus. The CSU Spur Campus consists of three buildings-Terra, Hydro, and Vida-located on the National Western Stock Show campus in Denver, Colorado. The Terra and Hydro buildings have 18-inch (45 cm) deep intensive American Hydrotech green roof systems installed for a total of nearly 10,000 square feet (~920 square meters) of growing space. Over the past two summers, we have also grown garlic, green chilies, beans, eggplants, jalapenos, apple trees, and several types of medicinal herbs. Between our Terra green roof and the Hydro rooftop agrivoltaic system (combined solar panel and rooftop agriculture), we have grown ornamental plants and crops for both research and donation to our local community food pantry, the GrowHaus.
Hydro agrivoltaic rooftop, Colorado State University, Spur Campus, Denver, CO. Photo: Brian Adams.
We began investigating the potential for growing medicinal herbs in rooftop agricultural systems due to the increasing demand and public interest in alternative health products. There are some fascinating insights to be gained from growing medicinal herbs in the harsh environment of rooftop agriculture. We wanted to evaluate if the survival, production, and potency of medicinal plants would be affected by the more extreme growing conditions on a green roof. The herbs that we have investigated so far are anise hyssop (Agastache foeniculum), lemon grass (Cymbopogon citratus), German chamomile (Matricaria chamomilla), lemon balm (Melissa officinalis), Tulsi/holy basil (Ocimum tenuiflorum), and ashwagandha (Withania somnifera).
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Herbs Evaluated
Anise hyssop (Agastache foeniculum)
Anise hyssop is an excellent green roof choice for aesthetic reasons alone, with its beautiful purple flowers, clumping growth habit and incredible fragrance. Anise hyssop has performed well on our Terra roof. When in bloom, its flowers are often covered by many species of bees. We have found that anise hyssop does well in partially shaded areas. We expect it to thrive in the deeper shade of the photovoltaic arrays. Like most mints, it is a perennial with an expanding root system. It will also reseed abundantly. Anise hyssop is harvested at the green roof substrate level. All above-ground parts of this plant are collected for medicinal purposes.
Self seeded German chamomile. Terra rooftop garden, Colorado State University, Spur Campus, Denver, CO. Photo: Brian Adams.
Lemon grass (Cymbopogon citratus)
Lemon grass has proven to be an excellent addition to our rooftop farm. It is fast growing and produces a wonderful citrus scent. Lemon grass is an annual plant in our region because it is native to tropical areas. However, lemon grass has proven to be very well adapted to the growing conditions that exist on a green roof. We have grown this species both on green roofs and in native at-grade gardens. The lemon grass on the green roofs has outperformed the lemon grass on the ground, especially in the early part of the growing season when soil temperatures are still low and root growth is slow. Lemon grass will provide at least two harvests per season in our temperate environment. Lemon grass tolerates shade. It has performed well under our rooftop agrivoltaic arrays. Because lemon grass is low maintenance, requires little supplemental fertilizer, tolerates most light conditions, and can be sold as both a medicinal and culinary herb, it is an excellent annual crop for rooftop agricultural production.
German chamomile (Matricaria chamomilla)
German chamomile produces the familiar daisy-like flowers - white petals with a yellow "button" in the center. The flowers are the only part of the plant that is harvested for its medicinal properties. German chamomile is like anise hyssop in that it is self-seeding if the flowers are not routinely harvested. Flowers can be collected relatively quickly using a blueberry rake. German chamomile has proven to be most productive when grown in full sun rather than shade. Most plants that bloom in the shade have a delayed bloom, and German chamomile is no exception.
Lemon balm (Melissa officinalis)
Lemon balm is an excellent choice for the low maintenance and high yielding production of medicinal herbs in an agricultural green roof. Like lemon grass, lemon balm can be harvested several times in a season and can be sold as a medicinal herb as well as a culinary herb. Like other members of the mint family, it requires very little maintenance. It spreads through its root system and reseeds. All of the above-ground parts of the lemon balm plant can be harvested. Its fragrance is excellent, giving off a lemony scent that contributes to the flavor of the herb. We have found that lemon balm, much like anise hyssop, thrives in shaded areas, but will also grow well in full sun.
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Harvested ashwagandha. Hydro agrivoltaic rooftop, Colorado State University, Spur Campus, Denver, CO. Photo: Brian Adams.
Tulsi (Ocimum tenuiflorum)
Tulsi is also known as holy basil. It has an aniseed flavor and basil aroma. It is a relative of other basils in the mint family. Green roof growing conditions are ideal for Tulsi as it will quickly grow to the size of a medium-to-large shrub and reseed profusely. Tulsi does best in full sun, but we have found it grows well in rooftop agrivoltaic systems. It can be harvested several times in a single growing season, similar to other medicinal herbs. Tulsi requires very little maintenance. It seems to thrive in the high temperature, well-drained conditions of a green roof. We have found that Tulsi has the largest yield of any herb.
Ashwagandha (Withania somnifera)
Ashwagandha, also known as Indian ginseng, is an evergreen shrub. It belongs to the nightshade family. In our hardiness zone it does not survive the winter. The ripe berries can be harvested and the seeds collected to propagate the following year. We have started this crop from seed in our greenhouse and have had 100% success with transplants. Ashwagandha is quickly becoming a very popular species for medicinal use to reduce stress. Ashwagandha is harvested for its roots and requires only a single season to reach maturity, unlike other herbs we have studied. While it thrives in full sun, ashwagandha will also perform well when shaded by solar panels. Ashwagandha grows rapidly in green roof systems. This makes it an ideal crop for agricultural rooftops.
Conclusion
Hydro agrivoltaic rooftop, Colorado State University, Spur Campus, Denver, CO. Photo: Brian Adams.
In summary, growing medicinal plants on agricultural or even ornamental green roofs alongside traditional crops is a good choice. Many of these species can be marketed and sold as culinary herbs. They can also be used as medicinal herbs for teas and tinctures. These are highly adaptable plants. They thrive in the conditions provided by green roof systems. These herbs require very little maintenance, reseed frequently, and can be harvested several times during the growing season. Most medicinal herbs provide flower resources for pollinators. They can also enhance the aesthetic quality of agricultural green roofs.
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Jennifer Bousselot, Ph.D., GRP is an assistant professor of horticulture at Colorado State University in the Department of Horticulture and Landscape Architecture.
Brian Adams is a Master of Science graduate student at Colorado State University and earned his Bachelor’s at Metropolitan State University in Denver, Colorado.
For more information go to Jennifer Bousselot: Green Roof Research - Horticulture and Landscape Architecture (colostate.edu)
