Green Roof Growing Medium: All To Know About

Green roofs are an excellent way to increase plant space, increase ecological diversity, and reduce storm runoff.

However, plants need a growing medium to thrive and this is where the growing medium comes into play.

While there are many different types of growing mediums available, not all of them will work well for green roof applications.

Because green roofs are different from traditional gardening that you would plant your shrub in the ground.

Instead, you have to work with the limited growing space while considering specific requirements like drainage, insulation, and protecting root membrane which is not a concern in a traditional garden.

For a traditional garden, if you have chosen the improper growing medium, you might lose your plant or get poor performance. A lot of times this can be fixed by adding more organic content, compost, peat moss, or soil. Because you are working on the ground you have a lot of options.

But with a green roof, fixing an improper growing medium is not so easy. Because the volume you are working is limited, a lot of times you can’t just add more growing medium to the mix to get the right balance.

Therefore, you’ll need to do your homework and research which green roof growing medium will be right for your specific project.

In this post, we’ll take a closer look at green roof growing mediums and discuss some of the most popular options available. We’ll also provide you with some tips on how to choose the right growing medium for your project.

What is a Growing Medium?

Plants need five things to grow: sunlight, water, proper temperature, air, and nutrients. Therefore, they need a growing medium in order to receive water and nutrients.

green roof medium

A growing medium is simply a material that aids plant growth by providing nutrients and water.

It can be made of organic, inorganic, or a combination of the two. The most important aspect is that the growing medium creates an environment conducive to plant growth.

Because growing medium also provides housing and stability for plant roots. Therefore, it’s important to choose a material that will complement the plants you’re growing and the environment in which they will be planted.

There are many different types of growing mediums available. However, not all of them will work well for green roof applications. Different types of green roofs have different requirements. Therefore, it’s important to plan ahead and research which growing medium will work best for your specific project.

Also, because green roofs are installed on a roof deck rather than directly on the ground, the medium must be able to meet the unique structural and drainage requirements of a green roof.

What characteristics does an ideal growing medium have?

The most common growing medium is a combination of organic and inorganic materials. It is uncommon for a single material to meet all of the requirements of an ideal growing medium.

That is why it is critical to know what you are looking for before deciding on a medium. Once you know what to look for, you can start evaluating different materials to see which combination and blend would be best for your specific needs.

Before we discuss the various types of growing mediums, let’s look at some of the most important characteristics of an ideal growing medium:

Lightweight

As we already know green roofs are typically installed on top of a building’s existing roof. Also, most roofs are not initially designed to hold the weight of soil and plants. If we add the weight of water that the medium will hold, the load can be considerably increased.

That is why the ideal growing medium should be light enough not to overload the roof deck. Choosing the right growing medium blend necessitates some engineering calculations to ensure that the weight of the medium and the amount of water it can hold are within the safe limits for your roof.

Therefore, if your green roof was not originally designed for your building, you should consult with a structural engineer to ensure your roof can withstand the extra load.

Also, in some cases, local building codes may have specific regulations governing the installation of green roofs in some cases. Check with your local building department to see if there are any special requirements for your green roof design.

Water Retention

We already discussed that all growing mediums hold water. This is what we call water retention that refers to the capability of a medium to hold water.

Certain plants require more and consistently moist environments than others. Therefore, if you’re installing a green roof with plants that require more water, you’ll need to choose a growing medium that has a high water retention rate.

Otherwise, you will have to water the plants frequently, which can be inconvenient and in some cases impossible.

However, water retention comes at a cost. The more water a growing medium can hold, the heavier it will be. If your roof deck can’t handle the extra weight, you may need to look for lighter-weight planting mediums and/or grow different plants that require less water.

Drains water

One of the important benefits of a green roof is that it helps to manage stormwater runoff. That’s why the growing medium must be able to drain water quickly and efficiently back into the drainage system.

If the medium doesn’t drain well, water will pool on the roof, increasing its weight, damaging building materials, and encouraging mold and mildew growth.

Therefore, the growing medium should be able to drain excess water without any problems.

Offers Aeration

The roots of plants require oxygen. The growing medium should be able to provide that air space for the root system of the plant. If the soil is too dense, oxygen will not be able to reach the roots.

This means that your plant’s root system will suffer, as will its health and performance.

Fortunately, growing mediums can be designed with a variety of materials to create the ideal environment for better aeration.

Nutrient Retention

Plants require nutrients in order to grow. That is why it is critical to select a growing medium that not only contains the optimal amount of desired nutrients but also has the ability to retain those nutrients over time.

If the growing medium lacks nutrients or loses its nutrient content too quickly, the plants will require supplementation. Because this can be both costly and time-consuming, it’s critical to select a growing medium that manages nutrients well.

So plant roots that are planted in a growing medium with high nutrient retention capacity can actually benefit from the extra nutrition provided by runoff irrigation or rainwater captured on-site before it washes down your drains.

Temperature moderation

Different plants prefer different temperatures. Some plants thrive in hot climates, while others prefer cooler temperatures.

The growing medium should provide enough insulation to protect the plants from extreme temperatures. If you live in a cold climate, the growing medium should be able to moderate temperature fluctuations and keep plants from freezing.

If you live in an area that experiences extreme heat, or if your green roof will be exposed to direct sunlight throughout the day, choose a growing medium with some insulating properties to help protect plants from damage caused by high temperatures.

Weed control

Another important factor to consider when selecting a growing medium is weed control. If you don’t account for the possibility of weed growth, you might end up with a green roof full of weeds.

Weeds can be difficult to get rid of because they compete with the plants for water, nutrients, and sunlight.

As a result, an ideal growing medium should have some weed control properties in order to maintain a healthy and appealing green roof.

Low Cost

Cost is always an important factor to consider. But we all know that “cheap” does not always equate to “good.” In most cases, you get what you pay for.

So it’s important to consider the cost of your growing medium carefully and not just look for low prices. Instead, be sure you’re actually getting the specific features you need for your green roof project.

For example, if you’re looking for a growing medium with high water retention but it’s more expensive, it might be worth the investment. However, if two growing mediums are equally priced and have similar water retention rates, weight, and nutritional value, you may want to go with the less expensive option.

Factory-Blend vs DIY Growing Medium

When it comes to growing mediums, there are two options: factory-blend and DIY.

Factory-blend growing mediums are pre-mixed and ready to use. They are typically made of a variety of materials that have been specifically chosen for their ability to retain water, drain well, and provide aeration.

They can also be designed to retain nutrients over time, eliminating the need to supplement your plants.

DIY growing mediums are made by combining various types of materials and usually necessitate some testing to ensure that the selected components work well together.

Some people love mixing their own growing mediums and modifying the blend to get exactly what they want for their green roof project, while others prefer the convenience and peace of mind that comes with using a factory-blend medium.

If you are going to make your own growing medium, make sure you choose a good mix that will provide all of the desired qualities for your green roof project.

If you’ve already decided on the plants you want to use, you should be able to find a growing medium that’s tailored to their needs. Alternatively, you may need to do some research and create the best combination of growing mediums for your plants on your own.

If you have a commercial green roof installation, you will likely need to work with a commercial green roof company that has the equipment and expertise to mix your growing medium.

Otherwise, you may end up with a growing medium that doesn’t meet your needs and could cause costly problems for your plants down the road.

What Materials Are Used as a Growing Medium?

There are an infinite number of material combinations that can be used as a growing medium. We won’t be able to cover them all in this post. Hence, we’ll only cover a few of the most common materials.

Soil

Soil is the most common growing medium, and it can be found in almost any garden or hardware store. Almost all green roof growing mediums are made up of some sort of soil.

holding fertile soil

There are many types and names for soils, but all soils have one thing in common: they provide nutrients that can be used by plants as well as some type of structure to help hold the plant upright.

However, while the soil is a great growing medium and is readily available, it is typically not a complete growing medium and requires supplementation to bring it up to the level required for a green roof.

Perlite

Perlite is a white granular substance with a neutral pH that is lightweight and non-toxic. It’s made of volcanic glass that’s been heated to the point where it pops like popcorn.

Perlite with green cup

Perlite is a porous material that absorbs water while also allowing water to drain freely, making it ideal for composting. It’s especially useful in hot climates because it keeps the soil and plants cool.

Perlite improves potting compost moisture retention and drainage, making it ideal for succulents and other plants that thrive in wet soil.

Vermiculite 

Vermiculite is a mineral that expands substantially when heated to extremely high temperatures. It is sold in sacks of brown-gold flakes at garden centers and can absorb up to four times its weight in water.

Mineral Vermiculite Samples

Vermiculite holds more water and nutrients than perlite, making it ideal for plants that need more moisture to thrive.

Vermiculite and perlite can be used simultaneously. They both work in similar ways, so their combination is appropriate for seedlings and cuttings.

Pumice

Pumice is an excellent soil amendment for regulating moisture and improving aeration in the garden. It’s a volcanic rock that’s been mined and crushed into fine, light-colored material.

Pumice Pebbles

Pumice can be used right away without the need for industrial processing, unlike perlite or vermiculite.

Pumice is not the same as pomace, which is the solids left over after crushing the fruit. Pumice is likely something you use on a daily basis without even realizing it–in cosmetics, kitty litter, water filters, and pencil erasers. The “stone” is what is used to create stone-washed denim.

Pumice has extremely few trace minerals and does not degrade. It enhances soil structure rather than adding nutrients. Pumice loosens heavy soil while also keeping the soil from compacting which improves drainage and aeration.

Its porous nature traps excess moisture. Pumice behaves like a sponge, absorbing water and retaining it until the plants want it. The water is then slowly released into the soil. Its one-of-a-kind design can cut your garden’s watering requirements by up to 35%.

Pumice can also be used as a beautiful, long-lasting, and healthy mulch. An addition to your compost pile to reduce odors and manage moisture, as well as mixed into the bedding of your worm bin.

Pumice can be mixed into potting mixes, composts, and soils. Adding as little as 10% pumice to any garden soil mix is sufficient to reap the benefits. If you are planting succulents creating a 50/50 mixture of pumice and potting soil is ideal.

You may either use it right away or soak it in nutrients and beneficial microorganisms for a few days before planting.

Simply soak your pumice in compost tea or worm castings for several days to allow the nutrients and beneficial microorganisms provided by these sources of organic fertilizer to permeate into the pumice.

Perlite, vermiculite, charcoal, rice hulls, and coco coir are all used for comparison purposes as pumice, and each has benefits and drawbacks when compared to pumice.

Because pumice is slightly heavier than perlite, it will not float to the top of your soil over time.

It is equally effective as perlite for water-sensitive plants such as succulents and cacti. Also, pumice does not hold as much water as vermiculite does. It also does not break down in the same way that coir and rice hulls do.

Limestone

Limestone is a kind of carbonate sedimentary rock that is available abundantly in nature. The majority of limestones form in calm, clear, warm, shallow seas.

Limestone Pebbles

The most common type of limestone is made up of the skeletal fragments of marine organisms such as coral, mollusks, and foraminifera.

It is largely made up of the minerals calcite and aragonite, both of which are crystal forms of calcium carbonate (CaCO3). Limestone is used for soil conditioning, pH balance, and alkalinity in gardening.

Limestone is a natural source of calcium for the plants which is necessary for healthy plant growth. It helps to buffer the soil and maintain a neutral pH level, preventing the soil from becoming too acidic.

It prevents magnesium from mobilizing in the soil, which is a common cause of plant deficiencies.

Limestone can be used as an effective pH adjuster in the garden. It is also used as a soil amendment to increase the alkalinity of your garden. Limestone can be mixed into potting mixes, composts, and soils.

Coco Coir 

Coco coir is a by-product of the coconut industry that would otherwise end up in landfills where it decomposes very slowly.

coco coir

Coco coir is made from the husks of coconuts that have been soaked in water and then ground into a fibrous material. Since it is a by-product of the coconut industry, it is a readily available material and environmentally friendly.

Coco coir is used as a growing medium for plants, and it has some advantages over other materials such as peat moss.

It retains water and nutrients well. It also has a high cation exchange capacity, which means it can hold a lot of positively charged ions that plants can readily take up.

It has a neutral pH level, making it suitable for most types of plants and soils. However, it does tend to be more alkaline than other growing media such as peat moss and perlite.

Unlike many organic materials, it is low in salts and therefore does not pose a threat of salt build-up in the soil.

Coco coir is also used as an amendment to increase the water-holding capacity of soils. When added to soil in high enough quantities, coco coir can help soils retain up to four times their original water holding capacity.

Rice Hulls

Rice hulls are the outer layer of the rice kernel that is removed in the milling process. They are a by-product of the rice industry and are available in large quantities at a low cost.

Rice hulls

Rice hulls have a number of benefits that make them a popular growing medium.

Firstly, they are effective at increasing the water-holding capacity of soils. They are also resistant to decay, so they will not break down easily.

Rice hulls have a high pH level and therefore make an excellent amendment for alkaline soils. They are a good source of organic matter and nutrients for plants.

Rice hulls have been shown to improve the germination and growth of plants. However, rice hulls also have some drawbacks.

They are lightweight and so they can float. So they are not suitable for use in hydroponic systems. They can be dusty and cause respiratory problems for people working with them.

Zeolite

Zeolite is a naturally occurring mineral that is used as a filter and absorbent. It has a high water-holding capacity and is effective at removing excess salts from the soil.

zeolite

Because of its high surface area and porosity, zeolite minimizes compaction, improves infiltration, and aids in the aeration of deep root systems.

For organic operations, zeolite is completely natural, and when decomposed with manure, it transforms into a natural fertilizing system.

Peat Moss 

Peat moss is a dead fibrous substance that occurs in peat bogs as mosses and other living stuff degrade.

Peat Moss harvests

The difference between peat moss and backyard compost is that peat moss is primarily moss, and decomposition occurs without the presence of air, which slows the pace of decomposition.

Peat moss takes millennia to grow, and peat bogs add less than a millimeter of depth every year. Peat moss is not considered a renewable resource since the process is so slow. The majority of the peat moss utilized in the United States originates from distant Canadian bogs.

The extraction of peat moss has generated a lot of debate. Despite the fact that mining is regulated and only 0.02% of the reserves are available for harvest, organizations such as the International Peat Society point out that the mining process emits massive amounts of carbon dioxide into the atmosphere. Also, the bogs continue to exhale carbon long after mining is completed.

Sawdust

Sawdust is the byproduct of sawing wood and other materials (plywood, boards, etc.).

Sawdust

Sawdust is a light substance. Sawdust has a bulk density of 220 Lb (100 kg) per cubic meter, and one-ton includes 9-10 cubic meters of raw material with an average moisture content of 8-15 percent (Table 1). This material is quite simple to work with.

Sawdust is a sterile substrate that is colonized by microflora as soon as it enters the soil. When it comes to organic material, the microflora that decomposes sawdust consumes nutrients from both the wood and the soil, depleting the latter of critical elements (the same nitrogen and phosphorus).

The components of natural wood sawdust do not cause allergies, and when burned, they do not generate dangerous gases.

However, it is important to remember that the above-mentioned components are typical of real wood, the quality of which influences the composition of sawdust. Sawdust, which is waste from artificially obtained boards that have been impregnated with glue and paint, is not suitable for use in horticulture.

Sawdust is named after the major types of woody crops, such as birch, linden, oak, chestnut, pine, poplar, coniferous, and so on.

On farms, any form of sawdust (from any tree species) can be used. But initially, several approaches must be used to lessen their harmful impact on soil composition.

This is the most widely available and least expensive raw material, with several applications in individual houses. Sawdust is used in the construction of outdoor structures, as well as for the insulation of walls, floors, and other structures.

Dense and heavy soils include black, clay, and loamy soils. Most garden plants like soil that is light, loose, breathable, and permeable.

When creating a greenhouse substrate or a soil mixture for seedlings, you can add up to 50% by volume of soil sawdust to increase the quality composition of this soil.

Before application, sawdust is mixed with semi-digested manure or added to mineral fertilizers, urea solutions, or cowpeas to ensure that it does not diminish fertility.

Wood Mulch

There have been mulch on the ground beneath the trees for as long as there have been trees growing in the forest. Mulch benefits both cultivated gardens and wild forests, and chopped wood is a great mulch.

wood chip mulch

The usage of wood mulch protects the environment because leftover wood is diverted to the garden rather than a landfill.

Wood mulch is inexpensive, readily available, and simple to spread and remove. It isn’t blown around by the wind like lightweight mulches. When it no longer looks good, compost it or work it directly into the soil.

Compost

Compost is organic matter that has been decomposed by microorganisms. It is the end result of the decomposition of organic matter like leaves, grass clippings, vegetable scraps, and animal manure.

compost in hand

There are different types of compost, and the quality varies depending on what is included in the mix.

It is an excellent addition to most green roofs. Because it provides nutrients and the micro-environment of the soil by introducing beneficial microorganisms.

Conclusion

Green roofs are important in the effort to reduce global warming. They provide insulation, reduce air conditioning costs, and improve the environment by trapping pollutants.

The type of growing medium you use is important for the plants’ health and your roof’s longevity. You can a combination of growth mediums to get the best results for your green roof.

When considering growth mediums, choose lightweight options that are rich in nutrients and have good water retention that also drains well.

Most importantly, you should consult with your horticultural expert or landscape architect for recommendations on the best growth medium based on the type of plants you want to grow.