Green Roof Facts – Plug vs Sedum

Introduction

Adrian Culling, Technical Manager for Blackdown Greenroofs has been working at Blackdown since 2008, and has priced a lot of green roofs in that time, but recently it seems that the majority of specifications that come cross his desk are for sedum mat – why is that? In the past Blackdown regularly specify and price for sedum plugs, both systems offer advantages and disadvantages, in this article he’s tried to summarise these as briefly as possible below.

Producers

It is very difficult for any grower to guarantee that the number of species within each square metre is supplied. The grower is more likely to state the mat will be made up of a combination of their listed species. Blackdown state that they have 6 core species and additionally 13 randomly sown species present in NatureMat®. Some sedum mat growers on the continent produce mat with as little as 4 species within it.

This is because some European and Scandinavian markets prefer to have fewer species within their sedum mat. With a plug plant system, the number of species can be clearly specified by m2. It is easier to introduce additional wildflower species into a plug system than to try and ‘inter-plug’ a sedum mat system.  Over-seeding a sedum mat system does not work, whereas, over-seeding a plug system will work i.e. over-seeding wildflower seed with sedum plugs.

Installation

Aspect

With a plug planted system the aspect of the roof i.e. its orientation and therefore the amount of sunlight or shade that the greenroof receives, can to some extent, be compensated for by the species of sedums chosen to be included on the roof. A blanket system does not offer this degree of flexibility when it comes to dealing with shaded areas.

Customization

Blackdown grow over 25nr different types of sedum and sempervivum in plug form – this enables Blackdown to customise the composition of planting on a greenroof.  Given sufficient time Blackdown can grow to order additional species of sedums – for further information contact us.

Blackdown would only recommend that a sedum plug system be installed up to a max pitch of 25 degrees, above this a sedum mat system should be specified.

Aesthetic

The greater variety of plug plants that are included within a plug system will only enhance the aesthetics of a green roof. This will help to prolong the flowering period and increase the visual interest of the green roof compared to NatureMat®, which has few species included within it.

Installation time & cost

A sedum plug system will take slightly longer to install compared to a mat system, however, the overall price should be cheaper due to the fact that there is less plant material being installed on the roof i.e. the GRO guidelines state that a mat system has a min of 75% plant coverage, whereas, a plug system will have between 5-10% coverage on installation.

Coverage

Blackdown NatureMat® typically has a sqm coverage of 90-95% upon installation. The establishment of a plug system will take approx. 6-9 months to get to 75-90% coverage.  However, this depends upon the month of planting, level of maintenance and irrigation applied.

Lifting and Offloading

No matter which system is ultimately chosen, the materials still needs to be lifted to the roof and distributed – and as each system has the same weight and depth there is little or no advantage when it comes of on-site services.

Establishment and Maintenance

Temporary Irrigation Temporary irrigation requirements will be the same for either system i.e. they’ll be required for a period of 6-8 weeks or until the plant material is fully established. However, if the system is correctly specified & installed, neither system will require additional irrigation once the plants are established.

Maintenance

Maintenance is often considered an optional extra – this is simply not the case. As with all plant material a little maintenance goes a long way to getting the best out of the system. Maintenance requirements will be the same for either system.

Conclusion

Blackdown Greenroof sedum plug systems offer a cost effect alternative to the NatureMat system whilst providing the benefits of increased numbers of species and better aesthetics. Both systems offer very tangible advantages to customers and specifiers, so perhaps we should give the humble sedum plug more room on our roofs.

If you have a project that may require our products please email any drawings, literature or enquiries to contact@alumascroofing.co.uk

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