Guest Interview with Nathalie Bauman on Hay Base Green Roofs

Organic Roofs Caught up with Nathalie Bauman, a Swiss green roof researcher specialising in urban ecology. Her work includes research into the suitability of green roofs as nesting sites for ground nesting birds and currently she is pioneering research into the hay base vegetation method technique. Oranic Roofs have already installed 200 m2 of roof space using this technique here in the UK during 2014 as it is a great solution to weight limitations when retrofitting green roofs.  Find out what she had to say about the benefits and challenges of using a hay base for green roofs.

Organic Roofs:  Thank you so much for giving us some time during your busy fieldwork season.  Please could you briefly explain the hay base system being developed in Switzerland for us?

Nathalie Bauman: The method is quite an old method – not invented by us. They used to do this all over Switzerland, cut the grass and the meadows and then use this material to put on the boarder of motorways to green it because it was easier to do and a cheaper method. And in the end it works very well because you have biomass to put on which also helps the seeds to germinate. So it was a method we knew from colleagues who were doing this and we thought ok lets try and do that with the ground nesting birds project. Which was research I was doing to see how we could improve existing green roofs (like sedum mats roofs and gravel roofs that do not have that much varied vegetation). So we had weight problems of course as you do when it is an existing roof, so it seemed like a good idea to try this as its light, its not that expensive (we had a limited research budget), and as we were doing research over 4 years we could test it. Is it worth doing it? Is it bringing something ect. So that is why we started putting hay up on the roofs.

OR: How does it differ from current roof systems?

NB: I prefer not to use the word system, when it is up on the roof and you think of the ecological processes then it is a system in itself but it is not a system that you would sell, it is more a vegetation methods technique.

Differences are that you can do whatever you like with the substrate, natural or technical or artificial, and then you decide I will seed, plant or both.  Well here you have a choice. For example, if you would like to try a cheaper option and nearby there is an organic farm, often farmers have a lot of this material so it is a kind of recycling. And then you put the hay instead of seeding and planting, but you can also seed and plant too so it mixes it up and can create a more diverse planting on the roof.

OR: So you mentioned the advantages of price, weight and recycling or ‘up-cycling’ waste from farm cuttings.  Are there any other advantages?

NB: Well the extra layer of biomass helps the growth of new vegetation through raising humidity thus creating better conditions for germination and establishment of the different plants.  Also sourcing material from an existing ecologically diverse meadow makes for good quality seed and a diverse locally appropriate seed bank, which will strengthen conservation at a landscape level.

OR: Are there and disadvantages to the hay base method and if so, what are they?

NB: I would say the biggest challenge, as with many things, is logistics.  It is best to use freshly cut hay but that obviously only happens during one season of the year and a green roof may be installed at any point during the year. Hay can be stored and is still ok to use after one year in storage, but then there are storage costs.  Another consideration is transport. If the farm you are getting the hay from is a long way away (which can often be the case with urban installations) then you will need to plan the logistics of transportation and this can incur further cost and a higher carbon footprint also.  This method really works best if a farm is near by.   In fact the choice of farm can be very important as, if it is not a dry meadow then the seed from lots of fast and aggressive grasses can start competing out other plants. It is very important ecologically to know the source of the hay in order to know the species composition you are ‘installing’.

As I mentioned before, the best approach is to combine techniques, the hay base seed bank and plug plants and extra seed, as you won’t get perennials with the hay base, just annuals.  But you can always just put the hay up and leave it and see what comes up!

OR: Finally how do you see this method progressing and contributing to the future of green roofing in Europe and further afield?

I see it firstly as another option to add to the portfolio of approaches developing.  It is a more organic option, which is important to some and may prove more important in the future as pressure on resources continues.  It also links in well with resource management as it is recycling waste.

Most importantly it is a great option for retrofitting when weight is a major consideration.  With straw, hemp, hay and a little bit of compost you can put more layers on a roof than just soil and so give the plants what they need with approximately half the weight.

OR: Great, thank you so much Nath and we here at Organic Roofs really support what you are doing and wish you all the best in your future work.  We will be following it closely!