While rooftop horticulture provides many advantages, including cooling buildings and reducing rainwater runoff, it is not without its challenges! Plant stress caused by the harsh conditions found on rooftops, such as high winds, intense heat, and drought, can be reduced by choosing the right growing media, and with the help of mycorrhizae.
Advantages of Rooftop Horticulture
Besides the convenience of having access to a private or semi-private green area for leisure, rooftop horticulture provides many advantages for city living. Green roofs do not only minimize smog by using up waste carbon gases in the atmosphere, which can cause respiratory problems and asthma, but they also lessen heat build-up during the hot summer months by shading the buildings’ roofs, thereby reducing cooling costs. The sun’s heat causes the free water in plants’ leaves to evaporate, cooling the air. Rooftop horticulture can also help reduce water runoff, which is an issue in cities, as plants use rainwater to grow.
Depending on the plants used on their green roof, gardeners can also have access to their very own locally grown food, at a short distance from home!
Challenges of Rooftop Horticulture
Environmental conditions can be very harsh on rooftops. High winds, intense direct sunlight, and lack of water are common. Therefore, it is important to find a growing medium with properties that accommodate both the harsh conditions and the usual requirements of a specific crop. And with the extra help provided by mycorrhizae, rooftop horticulture is more accessible than ever!
Choosing the Right Growing Media
Over several years of evaluation, it was determined that the most important property of a growing media used on roofs is a good drainage capacity. Other important properties include:
-Good aeration
-Good water holding capacity (without getting waterlogged)
-Resistance to decomposition
Peat-based growing media is a great option for rooftop horticulture since they are lightweight and provide high air porosity. It is recommended to amend the growing media with other materials such as a mineral aggregate or bark to add weight to containers if needed and to limit compaction which can cause plant shrinkage.
When amending a peat-based growing media for rooftop horticulture, make sure to take into consideration the added weight of the materials. For example, bark and calcined clay, or other lightweight clay products, should not exceed 30% of the volume of the growing medium.
Another type of growing medium that works well for rooftop horticulture is peat-chunk coir growing media. Available in compressed packaging (saving space in a city environment) and easy to handle, chunk coir-peat growing medium has a higher water holding capacity, dries out slower, provides good aeration, and resists compaction over time.
Compost and Urban Horticulture
While blending compost into a peat-based growing medium provides good water retention, it can alter its pH and add extra weight. When blending, it should not exceed 10% of the peat-based growing medium’s volume. Compost contains fine particles which can block the airspace in the substrate, causing a loss of aeration. Also, compost’s chemical and physical properties can be quite variable, resulting in a lack of constancy.
Soil and Urban Horticulture
Soil and topsoil are bulky, heavy, and inconsistent, which make them a poor choice for rooftop horticulture. The saturated weight of soil is more than four times heavier than a peat-based growing media. Soil has the potential to be contaminated with chemicals, pathogens and weeds, and, unless it is screened, it can contain rocks and large debris, adding weight, which is a concern for roof weight limit.
Mycorrhizae and Urban Horticulture
Mycorrhizal fungi help crops resist environmental stresses found on rooftops. Mycorrhiza (meaning “fungus root”) is the symbiotic and mutually beneficial relationship between plant roots and mycorrhizal fungi, which is known to improve growth and quality for a vast majority of ornamental and vegetable plants used in urban horticulture.
Mycorrhizae are microscopic beneficial fungi that form an extension of plant roots to capture nutrients and water that would otherwise be unavailable for plants.
How Do Mycorrhizae Work?
When fungal spores germinate, threadlike structures called hyphae penetrate plant roots. This is the beginning of the symbiosis. After the colonization, an extensive network of hyphae emerges from the roots, expanding the absorptive surface of the rootzone. This results in a better uptake of water and nutrients by the plant.
Elements like copper, manganese, zinc, and phosphorus are mobilized by mycorrhizae. In this mutually beneficial association, the plant supplies the fungus with carbohydrates in exchange for these nutrients.
Benefits of Mycorrhizae in Urban Horticulture
Horticulturists using mycorrhizae in urban settings can count on many benefits, including:
– Reduction of fertilizer application rates
– Decrease in plant nutritional deficiencies
– Prevention of wilting due to water stress
– Improvement of plant growth
– Resistance to salt toxicity
– Augmentation of fruits and flowers
– Lessened susceptibility to root-rot pathogens
– Better recovery and growth after transplantation
Conclusion
While urban horticulture comes with its specific challenges, using the proper tools, such as the right growing media with mycorrhizae, will help plants thrive in harsh growing environments. By following the advice provided in this article, businesses and city-residents can all benefit from having a green space which is environmentally friendly, a source of locally grown produce, and a nice place to relax!