A quick guide to the different methods used to establish plants on a green roof.
Having decided on your green roof planting pallet, it’s time to decide how you will establish those plants. There are a number of methods open to you, each one has its own set of advantages.
- Seeding
- Plug planting
- Vegetation matting
- Modular green roof trays
- Mixing methods for the best of all worlds
Your decision will likely be based on a number of factors
- Your budget
- Substrate Depth (trying to dig holes in a shallow substrate may result in damage to the waterproofing)
- Convenience and ease of establishment
- How quickly you want to see complete plant coverage
- The amount of time you can dedicate to planting
- Your capacity for weeding between plants until they are established
Seeding – Lower Cost
Seeding is undoubtedly the least expensive way to establish plants on a green roof. It’s also the least dependable. Conditions on a green roof are not always conducive to germination and it may take 12 months or more for a strong plant population to develop. And then of course you will have the wildlife to contend with. Many a horticulturist has been frustrated to see all of their precious seeds gobbled up by greedy birds.
When seeding a green roof, you may also find that additional species appear on your roof. Seeds dropped by birds or blown in by the wind will happily make themselves at home in that lovely substrate. If you are aiming for maximum biodiversity, this may well be advantageous. But do watch out for plants that are not suitable for green roofing, such as invasive grasses and/or tree seedlings. Be prepared to weed them out.
Whilst seeding is a cost effective way to establish biodiverse roofs, GRO do not recommend trying to establish a sedum roof from seed.
Plug Planting A Popular Way To Establish Plants On Green Roofs
Plug planting is probably the most commonly used method of populating a green roof. It’s cost effective, and whilst the actual planting out can be quite labour intensive, the young plants are normally quick to establish a strong root system.
With plug planting you have full control over the plant palette. Try to buy from a specialist nursery who uses a peat free growing medium. That way you will preserve the quality of the substrate.
In the early stages of establishing a living roof through plug plants, be prepared to weed in between the plants from time to time. As with seeding, opportunist plants will be very happy to enjoy the conditions that you have created on the roof.
To supplement the biodiversity of a plug planted roof – try seeding between plants. That gives you the option to use wider spacing between plug plants thus reducing the number of plants needed and helping to control costs.
Planting plug plants can be time consuming but it does allow you to tailor your plant palette to conditions on the roof. Photo credit to Bridgman and Bridgman
Vegetation Mats For Instant Visual Appeal
Speedy and relatively cost effective. Sedum blankets and wild flower matting will give you virtually full plant coverage in a matter of hours. However, unless you order them well in advance, you will probably not be able to specify which plant species are growing within the mats.
The rolls themselves are heavy – you will need mechanical lifting gear to get them onto the roof. And if they need trimming to size – for goodness sake sit them on a board to cut them (or do it on the ground). Accidentally slicing the waterproofing with a Stanley knife is not recommended!
Careful project management is imperative when installing vegetation matting. The mats must not be left rolled up or stacked for any length of time, particularly in warm weather. Plants deprived of sunlight will deteriorate very quickly indeed.
Modular Green Roof Trays – Simple And Convenient
Modular green roof trays seem to be a quick and convenient way to create a living roof. They are effectively interlinking mini-green roofs, each one with it’s own plant population, substrate and drainage system. Again, because they are pre-grown, you probably don’t get to choose the plant species unless you order 9-12 months in advance. However, there’s no reason why you can’t introduce a few plug plants of your own into the modules.
Pre-planted green roof trays like this one from GRO Members Harrowden Turf offer a quick, simple and convenient way to establish plants on a living roof.
Mix and Match – Combining Methods
There are no rules that say you can’t use more than one planting system on a green roof. In fact, using a combination of vegetation mats, plug plants and seeds will provide you (and the ecosystem) with the best of all worlds. You will have more control over the species, get fast coverage in some areas and be able to manage your budget.
For maximum interest on your green roof, vary the substrate depth across the roof and create a range of plant communities. You could have areas with shallow substrate with few plants where insects can bask in the sunshine.
A Word About Substrates And Irrigation
The long term success of any planting scheme, whether it be on the ground or on a roof, ultimately depends on providing the right growing conditions for your particular plant species.
Depth of substrate is crucial. The GRO Code Of Best Practice recommends at least 80mm substrate depth for a simple sedum roof, and a minimum of 150mm for a wildflower roof. But it’s not just depth that matters, different plant species thrive in different types of substrate. So it’s well worth talking to a specialist substrate supplier before specifying the depth and type of growing medium for your green roof.
Irrigation is also essential for the establishment phase of your green roof. Once the plants are well rooted, an extensive green roof should be reasonably self sufficient in water. There may be periods of drought where the plants seem to struggle, but, if you have chosen the right species for the conditions on the roof, Mother Nature will find a way to recover. Having said that, it’s well worth investing in an irrigation system – even if it’s only used occasionally. Remember though – the deeper your substrate layer, the less often you will need to irrigate established plants. (Young plants MUST be watered until they have rooted into their growing medium.)
Need help deciding how to establish the plants on your green roof? Talk to the team at GRO who can offer advice.
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