Establishing a green roof - vegetation blankets vs plug plants

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My Nan always used to say “there’s more than one way to skin a cat”.  Not sure what that meant but there’s certainly more than one way to plant a living roof.

Commonly used methods of getting plants to grow on a green roof include seeding, plug planting and vegetation blankets…..oh and leaving it alone to see how Mother Nature colonises the plant layer. 

I’m going to compare and contrast plug planting with vegetation blankets. 

What are plug plants?

They’re very small, junior plants that once planted out will grow and spread.  You may have seen bedding plants or vegetable plants being sold in the garden centres.  Because they’re small, they’re relatively inexpensive.

a single plug plant sitting in the palm of a hand

Plug plants are typically 4-5 cm tall.  On a green roof its usual to use 20-24 plants per square metre.
Photo is from gardeningdirect.co.uk

What are vegetation blankets?

If you’ve ever seen a lawn being laid using turf, you’ll understand the principal of vegetation blankets.  (Technically speaking, lawn turf is grown and harvested completely differently – but it’s certainly installed the same way as a vegetation blanket.)

Seeds are sown onto a matting system with a thin layer of growing medium.  They’re nurtured while they germinate and grow into mature plants.  Once the mat is almost completely covered with plants, it can be rolled up and despatched to site.  

cross section of sedum vegetation blanket

Sedum vegetation blanket.  This picture shows the carrier fleece, the growing medium and the dense layer of sedum plants.

Comparing costs

A quick internet search* tells me that plug plants in wholesale quantities cost around 35p each.  On a green roof, you’ll be needing 20 -24 plants per square metre so that comes to around £8.00 per square metre.

Enviromat sedum matting is around £30.00 per square metre + VAT and carriage

Meadowmat wild flower matting averages £10.00 per square metre + VAT and carriage

BUT remember that the plant layer is only one cost for green roof installation.  You also need to consider the cost of the growing medium, lifting materials on to the roof, the labour, the irrigation and, in the case of plug plants, regular weeding between plants while you wait for them to grow into the gaps.

Enviromat sedum matting doesn’t need any extra growing medium (but feel free to use it if you want to).  It just needs drainage matting at around £8.00/m2.

Plug plants and wild flower matting normally need 100 – 150 mm of green roof substrate beneath them, depending on the plant species chosen and on the availability of irrigation.

Storing the plants until ready to install

Vegetation mats – storage is a big no-no!  They MUST be unrolled on the same day as they’re delivered.  Preferably in their final position but if that can’t be done, lay them on the lawn or the drive or something with the plants exposed to sunlight and keep them watered until you’re ready to put them on the roof.

Plug plants – the trays take up a lot of room and can’t be stacked for any great length of time but provided the plants can all see the sun and you can keep them well watered, you can probably keep them on site for a week to 10 days before planting them. 

Comparing installation time

Vegetation blankets can be laid very quickly indeed – especially when you get the hang of it.  Simply manoeuvre the rolls into position and unroll them.  I’m very much an amateur but I can lay around 20 m2 (10 rolls) in an hour.  That doesn’t include lifting mats onto the roof but it does account for moving them around the roof, unrolling them and butting them up close together.  Someone who is physically much fitter than me (that’s most people!) could work a lot quicker.

Plug plants – I’ve never tried it, but as a rough estimate based on planting my summer bedding – 7 square metres an hour?  (about 170 plants) It’s less taxing, physically, but on a large roof I think I’d get very bored very quickly.

Comparing establishment time

Vegetation mats give you instant plant coverage or around 80 – 90%.  All you need to do is ensure they don’t run dry before the roots have settled.  They’ll need watering for 2 -4 weeks.

Plug plants take a little longer to get going.  On the other hand you are able to plant a design into your roof by grouping the species according to colour, flowering time, height etc.

If you are plug planting your roof, be prepared to wait up to 2 years until you have full coverage.  You may need to weed in between the plants during that time and you will certainly need to irrigate for the first few weeks after planting.

green roof created with sedum blanket

A newly installed green roof created with sedum vegetation blankets.  This will need feeding every year and possible a minimal amount of weeding - other than that its good to go.

 

In summary:

The choice is yours.  Plug plants give you more design options but cost more in labour.  Vegetation mats are instant and they’re easy to install.  Over to you………

 

 More about Designing a living green roof

 

Great plants for green roofs

 

*(today is 9th May 2016 so if you’re reading this in the future these prices may have changed)