Green roof planting ideas

The long-term success of a green roof is heavily dependant on the choice of plant species, the method of establishing the plants and their maintenance regime.

Sedum green roof overlooking a great view

Right plant, right place

Let's take a look at that age-old saying "right plant! right place"

The plant pallet available to Gardeners today is vast. Absolutely vast. The average horticultural retailer will offer plants from every continent and every climate. There are annuals, biennials and perennials. Hardy plants, half hardy plants, woody plants, herbaceous plants, foliage plants, succulents, herbs, grasses ... The list goes on. So how does a self-respecting architect go about helping a client select a suitable plant pallet for a green roof?

The first step in selecting plants is to look at the conditions they are to grow in. When designing a new building with a living roof, most of the growing conditions can be specified. When retrofitting you are somewhat restricted to adapting the roof to suit the building and it's environment.

What do green roof plants need?

Plants are living things, and like all living things they need 3 basic things;

  1. Water - in the right quantities for the species. Too little and they wilt, too much and they drown.
  2. A source of food. Plants harness energy from sunlight to combine basic chemical elements the air and from their growing medium to make their own food.
  3. Air. Every living thing needs air to breath and most like it to be within certain temperature parameters. Unlike humans and animals, plants cannot move to a warmer or a colder place if the temperature doesn't suit. So choose plants to suit the temperature conditions on your roof ..... Or be prepared to re- plant it when they expire.
    1. As an architect designing a living green roof, you must find a balance between what the plants need and what the building can support.

Living green roof in poor condition

At some point, the sedum plants on this roof didn't get what they needed

Green roofing essentials

  • Daylight: It's a basic requirement for plants. If your roof is in deep shade for most of the day, a green roof is not a sensible option
  • Drainage - to ensure plants are not waterlogged
  • Water retention - To supply basic requirement for water. For particularly thirsty plant species, on very exposed roofs, steep slopes or when the substrate layer is too shallow for the plants' root system, an irrigation system may be needed.
  • Growing medium. This is what will anchor the plants to the roof and supply vital nutrients. NEVER even think about using topsoil. Only use a specially engineered green roof substrate. It's lighter than soil, the partical sizes mean it won't erode or become compacted and the reduced proportion of organic matter mean it won't oxidise.The depth of growing medium will be determined by the load bearing capacity of the roof
  • Plants - more on that later.
  • Maintenance - bear in mind that whatever planting scheme you go for, it WILL need maintenance. That means safe access, and managing client expectations.

How deep should the growing medium be?

A green roof is essentially a large, somewhat flattened version of a plant pot. The plants growing in it have a finite space for their roots. The depth of the growing medium on a green roof is dictated by the load bearing capacity of the building. As a rule of thumb, the deeper the better because it will offer better insulation, stormwater attenuation and plant diversity. But where weight is an issue, a sedum blanket build-up is the least heavy of all green roof systems.

When plants become pot bound, they either die, perform poorly, or the roots attempt to escape. The very last thing you need on a green roof is for the plants to damage the waterproofing so:

  • always choose plants with non- invasive roots
  • make sure growing medium is a suitable depth for your chosen plant pallet

A list of plants and substrate depths can be found here

Suitable plants for green roofing

Conditions on a roof are often more extreme than conditions on the ground. When the weather is hot, a roof is hotter. when it's cold out there, the roof will be much colder. On a sustainable, easily managed green roof, the plants will also need to cope with high winds and minimal maintenance. Choose plants that are;

  • Drought tolerant
  • Frost hardy
  • Heat resistant
  • Perennial (ie they grow back every year so you don't need to replace them)
  • Self perpetuating (ie they will set seed, spread via the roots, or make new shoots to fill in bare spaces
  • Have a sensible growth habit ie (won't outgrow their situation)
  • Have low nutrient requirements (green roof growing medium is not rich enough to support hungry plants)
  • Have non-invasive roots
  • Have achievable maintenance requirements
  • Are suited to the levels of light and shade on the roof

Your green roof plants should also enhance their environment. In which case they must be;

  • Aesthetically pleasing and in keeping with the building. Particularly if the roof is overlooked
  • Valuable to local wildlife. Ie provide food, shelter and resting places for insects and birds
  • Where BREEAM points are an objective, green roof plants need to be acceptable to the onsite SQE

 sedum flowers in summertime

A bumblebee enjoying sedum flowers

Establishing plants on a green roof

An important factor in the specification of green roof plants is establishment. How will the plants get to be growing in the substrate you have provided?

Apart from self-colonisation; where the substrate is left bare and Mother Nature is allowed to create her own plant pallet over time, there are four different ways of establishing plants on a living green roof

  1. Seeding
  2. Plug plants
  3. Mature plants
  4. Vegetation blankets

As an architect, you need to balance your clients wishes with the real life practicalities. What is their budget? Are they expecting instant greening or are they prepared to wait while the plants get established? Do they want the neat, contemporary look achieved with sedum blankets, or would they prefer a shaggy wildflower meadow on their roof? Are they expecting instant greening or are they prepared to wait while the plants get established?How will you lift the plant material onto the roof? Who will do the planting?

This table compares the different types of plant establishment so that you can strike the right chord with your clients

Establishment Method Cost per square metre Time to maturity Effort needed
Seeding Low up to 2 years will need regular weeding and irrigation
Plug planting High - labour intensive up to 2 years Will need regular weeding and irrigation
Vegetation Blankets Moderate Instant coverage Irrigation for 2-4 weeks
Mature plants High - labour intensive Up to 1 year Irrigation for 2-4 weeks

Remember too, that the simplest, fastest maturing system will likely bring the quickest at paying back its carbon cost. Particularly if the plants are grown as locally as possible to the site.

Substrate depth for green roof plants

PLANT COMMON NAME SUBSTRATE DEPTH
Achillea millefolium Yarrow 10cm +
Allium schoenoprasum Chives Up to 10cm
Lavandula angustifolia Lavender 10cm+
Lotus corniculatus Birdsfoot trefoil up to 10cm
Sedum album White stonecrop as little as 2cm
Sedum matting Enviromat is a popular brand No extra substrate needed

 

For more information on designing a living green roof, download our Designing for Maintenance pdf

 

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