How do green roofs help biodiversity in urban areas?

Urbanization has led to the fragmentation and loss of green spaces, which is detrimental to biodiversity.

Green roofs can help reverse this by serving as refuges for certain species of plants and animals.

Green roofs help pollinators, which are needed for plant reproduction. They also absorb rain and reduce stormwater runoff and temperatures (especially when covered with deep soil or vegetation), thus mitigating the urban heat island effect.

For these reasons, green roofs are becoming more popular in urban areas.

Knowing the many benefits of green roofs, scientists are looking for ways to make them even more valuable. They want to leverage green roofs as a tool to bring back biodiversity in urban areas.

Because we have already lost so much biodiversity in urbanized areas, it is important to focus on restoring them.

This can be done by introducing plants that harbor a wide variety of pollinators and plants that provide food and shelter for birds, mammals, reptiles, and amphibians.

There are many great strategies that urban planners, building designers, and homeowners can use to increase biodiversity on green roofs.

We have gathered some of the most important ones that may inspire you to create a green roof with as much biodiversity as possible.

Select the right plant species

Green roofs can support a wide range of plant species, but not all plants are suited for the conditions on green roofs.

For example, native plants that grow in a region’s natural biomes are more likely to be able to successfully grow on a green roof. Also, plant species will need to be chosen carefully if they are likely to grow too large or spread easily.

When choosing plant species, it is important to know the soil conditions on the green roof (which may vary depending on the depth of the substrate or whether it is a rooftop garden).

You should also be aware that certain plant species may attract pests that are not desired.

Furthermore, it is important to choose native plants that will not outcompete existing vegetation or disturb the local ecosystem.

Select the right substrate depth

Green roofs can be created on many different substrates. These can range from clay and sandy soils to artificial substrates. It is important to choose the right substrate depth so that your plants will survive and grow.

For example, if you want to grow plants that will need deeper soils (such as trees), you will need an intensive green roof.

However, if you want to make use of the space with plants that will not need deep soils, you can have an extensive green roof.

The depth of your substrate will determine the types of plants you want to grow. We have a dedicated post comparing both types of green roofs you can find here.

Choose the right planting dates

Planting dates are important because they will affect when the plants will grow and when they will flower.

It is important to know that some plant species can only flower during a certain time of the year and will need to be planted at the right time.

Planting dates also matter if you want to maximize pollination on your green roof.

This is because certain pollinators only fly during a specific period of time. You can find more information on the best planting dates for pollinators here.

Certain plants will need to be planted in their pots or containers until they are well established (usually for at least 1-2 seasons).

This is because the soil on green roofs will not contain all of the nutrients, microorganisms, and organic matter that plants normally grow in.

This problem can be solved by planting native plants that do not need much fertilizer and that can obtain nutrients directly from the soil.

Select right sowing rates

It is important to have enough plants on your green roof so that they are able to pollinate each other.

This will reduce the possibility of pests or plant diseases, which may be more common if you do not have enough plants.

Ideally, plants should be spaced so that they are able to cover the entire surface of your green roof.

However, it is also important not to saturate your substrate with plants so there is enough sun for all of your plants to grow.

Planting densities will depend on the type of plant species you want to grow and the substrate depth. Also, they may be affected by whether the plants need to be planted in their pots or containers until they are well established.

Have an irrigation plan

It is recommended that your green roof should be irrigated according to a plan.

This plan should not only include how often your green roof will be irrigated but also what types of irrigation it will use (such as drip irrigation).

It is important to remember that different types of plants will need different amounts of water per week.

A good strategy would be to irrigate your green roof with a drip irrigation system that is placed underneath the substrate and distributes water evenly.

Another strategy would be to place a sprinkler on top of the substrate and water your green roof in this way. This method is less practical because the water may run off instead of being absorbed by the substrate.

Choose the right planting techniques

Green roofs are usually planted with different techniques that will help reduce stress and improve plant establishment rates.

These include direct drilling into the substrate, seeding directly onto the substrate, or transplanting plants into pots.

Different plants may need different planting techniques because of their size, substrate conditions (e.g., clay or sandy soils), or the depth of the substrate.

However, choosing the right planting techniques can also make sure that the plants will establish more quickly.

For example, transplanting plants into pots can sometimes be a better choice if their roots will be damaged if they are planted directly into the substrate.

Create different planting zones

Planting zones are a great way to separate plants. This will make it easier for you to individualize your green roof and create a natural habitat.

You can support different plant types with different substrate depths by separating them into different planting zones. This way you will be able to grow taller plants in one zone and smaller plants in another zone.

This is also a great strategy if you want to separate pollinator-friendly plants from those that need wind protection.

Create habitat corridors

Habitat fragmentation is one of the reasons why there are so few pollinators in urban areas. Green roofs can be used as corridors to provide pollinators with access to different habitats.

You can achieve this by planting perennial flowers, hedges, and trees that connect two or more green roofs or natural areas.

Creating habitat corridors will not only increase the number of pollinating insects on your green roof but also promote biodiversity throughout the city.

Create a bird-friendly roof

Birds are also affected by habitat fragmentation, and green roofs can be a way to increase their population.

Some species of birds may even choose your green roof as a nesting site! It is therefore important to create a bird-friendly rooftop that will provide the right conditions for these animals’ survival.

For example, you can plant a taller zone with trees and shrubs. This will provide birds with shelter from wind and rain while also giving them a place to hide from their predators.

A green roof can thus be a safe haven for both insects and birds, increasing the diversity of your rooftop habitat.

To attract birds, green roofs should include berry-producing shrubs and trees, such as American holly or viburnum.

Birds also benefit from perching points created by planting small trees or providing old tiles or boards on the roof.

Integrate into the surrounding landscape

It is important to integrate green roofs into the surrounding landscape.

Doing so will make the green roof more valuable for local wildlife and it will also reduce maintenance costs for building owners because they can share resources.

The main thing to consider when selecting and designing a roof garden is making sure to choose plant species that can thrive in the local climate and will not introduce invasive species to the area. It is also important to avoid plants that are known allergens.

Host variety of plant species

To maximize biodiversity, green roofs should include a variety of plants with different heights and textures.

This will attract a greater number of species and will create a more complex ecosystem.

Include nectar-rich flowers

Nectar-rich flowers (e.g., perennial flowers with an extended flowering period) should be included in green roof vegetation to attract bees and other pollinators, which are essential for seed production.

Flowers should be planted in random patterns rather than rows to increase available microhabitats for insects.

Plant species with tubular flowers, such as Larkspur Delphinium spp., are especially good because the shape of their flowers ensures that small bees can access pollen by crawling through the flower tube.

Avoid invasive species and nonnative plants

Invasive or noxious plant species should be avoided when planting green roofs to minimize the spread of these plants into nearby natural areas.

Although it is generally assumed that green roof vegetation can’t survive long if it’s not regularly watered, some plants are able to establish small populations on green roofs that can eventually spread into natural habitats.

Use Weed-suppressing groundcovers

Weed-suppressing groundcovers help prevent invasive weeds and grasses from growing and can protect grass seeds and natural vegetation from being scoured or buried by soil erosion.

Create shady & Sunny spots

Insects are more active in shaded areas, which can be created by growing smaller plants underneath taller vegetation.

Place taller vegetation in the center of the roof to provide a greater canopy of shade for insects that prefer shaded habitats. Plant vertical layers with different microhabitats (e.g., sun and shade), which increase biodiversity.

Create living walls to maximize biodiversity

Green walls can further increase biodiversity by increasing microhabitats for invertebrates.

Vertical green walls should have a greater number of nooks and crannies in the vegetation, including grasses with different heights and textures.

Incorporate logs to provide additional habitat

Recycled building materials such as old tiles, bricks, and timbers should be used to create a rough ground surface.

This provides greater habitat for invertebrates because it creates microhabitats under the vegetation that are insulated from temperature fluctuations.

Logs can be placed vertically in corners to create shaded microhabitats for insects that prefer shady conditions.

Avoid the use of pesticides & herbicides

Insects that are beneficial to the surrounding habitat, such as bees and wasps, can be killed by pesticides or herbicides.

These chemicals don’t only harm or kill the targeted insect species, but they can damage other insects that are not the target of the pesticide.

Therefore, it’s best to avoid using pesticides and herbicides as much as possible. If they are absolutely necessary, consider using natural pesticides that have less impact on surrounding insect populations.

Incorporate water features

Water is very important for the survival of many types of plants and animals living in urban areas.

For example, many pollinators will visit plants that have water available. This can be simple as a birdbath or a place where animals can drink from.

Water features, such as small ponds or birdbaths, can decrease the amount of surface runoff and increase biodiversity. They provide an additional microhabitat for insects that prefer to live around water.

Incorporating water features take necessary measures against mosquitoes. Mosquitoes can breed if water features are not provided with mosquito-repellent plants, such as citronella grass.

Mosquitoes can quickly breed near stagnant pools of water, so it’s best to avoid creating these types of water features. Thus circulating water will be beneficial to prevent mosquitoes from breeding.

Placing water features on your green roof can also provide homes and shelter for certain creatures such as lizards and frogs.

Consider further green infrastructure

Green roofs are not the only way to create green infrastructure. They can be combined with other green infrastructure (e.g., rain gardens, permeable pavements) to maximize biodiversity.

Therefore, green roofs should be considered as part of a greater green infrastructure plan to maximize biodiversity.

Conclusion

Green roofs are an important form of urban ecology. They provide many benefits to the local community, such as reducing stormwater runoff and the urban heat island effect, by decreasing surface temperatures.

They also increase biodiversity in urban areas by creating habitats for invertebrates that live on or close to green roofs.

However, creating a truly diverse green roof requires planned design and consideration for the different habitat needs of local insects.

This includes creating vertical layers, incorporating bird-friendly plants, maximizing wildlife exposed surface area, avoiding pesticides and herbicides use, using water features with care, and considering additional green infrastructure.

Finally, creating a great green roof is not truly meaningful unless communities or at least the building occupants can enjoy it.

Therefore, they should be designed to provide people with a space to relax with scenic views.