Great plants for green roofs

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A living green roof can be a real asset to biodiversity, particularly in urban areas. There are many kinds of green roof on many types of structure all over the world and the trend for greening our buildings is fast gaining momentum as mankind realises just how vital plants are for our survival.

sedum green roof seen from an upstairs window

A living green roof is so much nicer to look at than plain old waterproofing

So what can you plant on your roof to make it green?

The plant species that will live happily on your roof depend on several factors. In this blog we'll look at each one in turn and suggest ways in which you can do your bit to preserve our amazing ecosystems (and make your building look good)

  1. Strength of your roof
  2. Aspect
  3. Access to the roof
  4. Type of growing medium
  5. Depth of growing medium
  6. What you can afford
  7. Time and budget available for maintenance

Lets start with the basics: What is your roof like?

Waterproofing

A green roof sits on top of the waterproofing. It won't damage the waterproof membrane, but it won't fix or mask any existing problems with it. Make sure your waterproofing is in tip top condition before you even consider creating a living roof.

man heat sealing waterproofing

Don't even think about installing a green roof unless your waterproofing is in tip top condition

Strength

A green roof is like a giant-sized (but flat) plant pot sitting on top of your building. You know how heavy houseplants in pots can be - the bigger and deeper the pot is, the more it weighs. Your green roof buildup will put extra loading on your roof. Can it take it? Best to check with a surveyor or a structural engineer before going any further. Oh - and get a figure for the loading capacity. With careful plant choices, there are ways of making a lighter green roof but you need to know your maximum payload before you start.

Light and shade

When you go to the garden centre, most plants have a label on them to tell you if they like sunshine, shade, wet soil, dry soil, frost protection etc. No matter whether they're destined for a container, a border or a roof. Those are the conditions they like.

green icon with sun peeping out from behind a cloud

Icons like this on a plant label help you to know what conditions the plant prefers.

You need to know if your roof is shaded by other buildings or by trees. Make some observations. If it has low light conditions for more than half a day - you need a planting plan for shade. If it's in bright sunshine, look out for plants that like not only sun, but dry conditions too. The same applies for really exposed roofs, high winds will dry the plants and soil out very quickly. Choose plants that don't need lots of irrigation (unless of course, watering plants is your favourite passtime)

Accessability

roofer climbing ladder with rolls of waterproofing on his shoulder

If a building is just one story high it might be OK to use a ladder for access but that's not ideal for higher buildings or when there are things like growing medium, sedum rolls or trays of plants to lift onto the roof.

Sounds crazy doesn't it? Especially if you're thinking about an extensive green roof where few people will walk on it except to carry out an annual maintenance inspection. But. How will you get the growing medium, the drainage mats, the plants and any other materials on to the roof? How will you get up there to water it for the first few weeks after installation? Looking forward, depending on the style of your planting you may have to access the roof to add fertiliser, trim, prune and remove cuttings, replace plants, inspect your beehives. Can you carry stuff on and off the roof? safely?

Now we get to the good bit, the plants

There's more to choosing green roof plants that just looking at a catalogue and picking out what's pretty. It's all about what will survive, what will look good, what you can care for and what you can afford. Lets think about that a bit more........

 bright white flower of meadow saxifrage

Meadow saxifrage.  A UK wildflower that does well on green roofs....even if there's a shallow layer of growing medium.  Look for delicate nodding blooms in April and May.

Which plants will survive on your green roof?

We've talked about light, shade, wind and water but we've not mentioned growing medium. The depth of the growing medium will determine the number of species you can choose from. And the loading capacity of the roof will determine the maximum depth of growing medium that your building can support.

A shallow growing medium 20mm or so will happily support sedums, sempervivums and some alpine plants. Some wildflowers, can thrive in 50mm of growing medium but 100mm will give you more choice. 150mm allows you to grow a lawn and use the roof for leisure (if the access is good enough) for trees etc, talk to a specialist.

Always used a specialised green roof growing medium (aka substrate). Topsoil is too heavy for a roof, it has tiny particles that can be washed away by rain, it compresses easily and needs to be dug or cultivated from time to time - which is a dodgy thing to do near a vulnerable waterproof membrane!. Substrate may cost a bit more but believe me, its an investment.

Water:If you want to grow plants in a shallow growing medium, you need a supply of water. Remember? a green roof is just a giant plant pot? What happens when you neglect to water your pot plants? I rest my case. For some species, the UK rainfall is enough to keep them going. Others need a little more TLC. Especially if your want your roof to look lush and vibrant all summer long. For an easy life, choose drought tolerant plants. Sedums are commonly used on green roofs for good reason. Other good choices include mediterannean-type herbs like thyme; moorland plants like heathers; houseleeks, alpines, grasses, or wildflowers normally found on chalk downlands.

Form and structure:Plants that fare best on green roofs generally have small leaves, are either low-growing or flexible enough to bend in the wind, roots that offer good anchorage but don't threaten the waterproofing (avoid thugs like bamboo), are perennial (so you don't have to replace them every year) or really good at seeding themselves. Look out for winter hardiness too. Things like banana plants, castor oil plants and other big leaved beauties will be spoilt by the wind. If you simply must have them, then you need to protect them.

man applying fertiliser to living green roof

Whatever style of green roof you choose, it will need maintenance.  In this picture a neglected sedum roof is being revived with a granulated fertiliser.  Although sedums are drought tolerant and don't need regular irrigation its important to have a water supply on hand.  This fertiliser needs to be watered in - if not it will scorch the plants.  

Care and maintenance: If access isn't good or if you're not keen on gardening. Go for something easy. I know I keep banging on about sedums but easy maintenance is one of the reasons they make such great green roof plants. Wildflowers tend to be pretty independant too. You could think about something like Meadowmat. It needs at least 100mm of growing medium but is easy to install and will look after itself for most of the year. It'll just need one big tidy-up a year and you don't need to be a horticulturist to do it.

Money, money, money

What does a green roof cost? Depends what you want really. To get the costings, and assuming you don't need to replace the waterproofing, you need to price up drainage mat, edging, growing medium, plants and labour. You may also need to factor in the cost of a cherry picker or a crane to lift materials onto the roof.

In general, sedum matting is probably the least expensive option. Wildflower matting is less expensive than wildflower plants because there's less labour involved in installing (planting) it. Seed is the cheapest - but you will need to wait a long time for it to grow and mature.

If you need a quote for green roofing materials, the Enviromat sales team can help. Call them on 0333 456 4526

 

Further reading

Top Ten Green Roof Plants


Designing a green roof with low maintenance


CPD - green roof design talks for professionals