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Criteria for Growing Media for Roof Top Gardening

Thursday, September 7, 2023 | Susan Parent

In our previous article "Urban Gardening - A Growing Trend", we discussed the basics of urban agriculture with a focus of growing plants on roof tops. In this article, we will discuss the ideal growing medium and properties required for roof top gardening. Growing on a rooftop or other outdoor areas of a building presents a challenge due to intense summer heat, lack of water, high winds, etc. Due to the harsh conditions, the growing medium has to provide properties to accommodate these conditions as well as respect the requirements of the crops grown.

When selecting a growing medium for a roof top garden, consider average rainfall in the summer months, average high and low temperatures during the growing season and the types of plants to be grown and their needs. If you’re in an area with a lot of humidity and/or rain during the growing season, you would need a media that dries out fast; but in a dry environment it is better to use a growing medium that holds water to minimize drought stress on plants.

During the past several years the performance of growing media on roofs has been evaluated in Europe, especially for green roofs and it was determined that the most crucial physical property the growing media should have is good drainage. This is quite a challenge for a growing medium to have good drainage and simultaneously good water retention. According to Latshaw, the ideal growing medium should have the following properties:

• Good drainage and aeration
• Good water holding capacity (without getting waterlogged or heavy)
• Good cation exchange capacity
• Resistance to decomposition or biodegradation
• Lightweight, but sturdy (resists shrinkage and wind displacement)
• Stable (to support plants)

Soil

In terms of handling and for practical reasons, soil alone or topsoil for roof gardening can be difficult to work with in comparison to manufactured soilless media. Not only is it bulky and heavy, it can have potential chemical and pathogen contaminants and, if not screened, may contain rocks or debris that may make the product inconsistent. Soil’s saturated weight can reach 120 lb/ft³ (7.5 kg/m³), which is far too high for roof gardening. The targeted saturated weight of roof top growing media should be 48-65 lb/ft³ (3-4 kg/ m³). 

Peat-based growing media

Peat-based, soilless media used in nurseries or greenhouses would work better as they have high air porosity and saturated weights of only 55 lb/ft³ (3.4 kg/m³) (see table below which presents the saturated weight of various materials). However, peat-based growing media can dry out fast and compact over time. Although it will not help reduce the dry down time, it is best to amend a peat-based growing medium with a mineral aggregate or other materials, like bark, to limit shrinkage due to compaction during the growing season. Although bark is a great component, it would add weight, so it should be limited to no more than 30% of the volume of the growing medium. Calcined, expanded or other lightweight clay products also limit compaction and improve stability, but likewise should not exceed 30% of the volume of the growing medium due to the extra weight.

Dry and saturated weights of growing medium components:*

*Weights are approximate and may vary with different products.

Chunk coir-peat growing media

Another type of medium that would work well for roof top gardens are peat-based growing media with chunk coir. Chunk coir-peat growing media offer good aeration for plant roots, compact less over time and do not dry out as fast as peat based media. They are easy to handle and are available in compressed packaging, so they take up less storage space than loose-filled bags. Chunk coir-peat growing medium can also be amended with clay aggregates for optimum durability for gardening. Premier Tech Horticulture has a chunk coir growing medium, PRO-MIX HPCC MYCORRHIZAE that provides good drainage and porosity for long term growth in roof top gardens. It also contains MYCORRHIZAE, a natural active ingredient proven effective for plants in stressful environments by enhancing uptake of water and nutrients to reduce the effects of stress on plants.

Compost

Blending compost into a peat growing medium will provide good water retention on hot days when plants are using up a lot of water, but compost has fine particles that can block the airspace in growing medium.  It also increases the weight per cubic foot of the growing medium and typically has a pH near 7.0, so it can increase the pH of the growing medium. The ideal pH for most plants should be at or below 6.0, especially for petunias, calibrachoa and blueberries. If compost is used, it should be no more than 10% by volume in order to minimize loss of aeration, weight and pH change.  

Keep in mind when selecting a growing medium, it is important that it has high air porosity and good water retention throughout the growing season. Depending on the plants grown, most will perform best in a blend of a peat-based growing medium with lightweight clay aggregates, coir chunk or heavier material to help support the plant throughout one or more gardening seasons.

For more information about growing media, please feel free to contact your Premier Tech Grower Services representative.

Ed Bloodnick Horticulture Director South Eastern US, Japan and Overseas	Nathan Wallace-Springer Horticulture Specialist South Eastern US	Lance Lawson Horticulture Specialist Western US and Western Canada	Victor Brantly Horticulture Specialist Central US
Troy Buechel Horticulture Specialist Eastern and North Eastern US	Susan Parent Horticulture Specialist Eastern Canada and North Eastern US	Jose Chen Lopez Horticulture Specialist Mexico and South Western US

References:

• Latshaw, K, J. Fitzgerald and R. Sutton. 2009. “Analysis of Green Roof Growing Media Porosity”. RURALS vol. 14 (Issue 1) Article 2 (University of Nebraska, Lincoln)
• Antoine Trottier.  2007. « Toitures végétales: implantation de toits verts en milieu institutionnel ».  Étude de cas : UQAM. 80p.
• SPSQ.  2013. « La serriculture sur les toits en milieu urbain. Perspectives de développements dans le contexte québécois. » 139 p.

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