Hint: Read all the way to the end for a natural replacement to SAPs.
Polymer water absorption crystals have been a popular way to help keep plants watered in containers, hanging baskets and even green roofs. There are many websites selling this product, such as Watersorb. Even other vendors can be found by Googling the phrase "polymer water adsorption crystals".
Yikes! These crystals can be expensive. $15.00 per US pound or $26 per three US pounds.
How could one afford to use these on a green roof at this price?
According to one of the most convenient information sites, Wikipedia, these crystals are commonly referred to as super-slupers.
The Super-absorbent polymers, also know as SAP's are created by cross-linking and chemically interacting polyacrylamide copolymer, ethylene maleic anhydride copolymer, cross-linked carboxymethylcellulose, polyvinyl alcohol copolymers, cross-linked polyethylene oxide, and starch grafted copolymer of polyacrylonitrile to name a few, according to the Wiki article.
OK - now for our suggestion. This is only a suggestion. The process is working quite well for us here in the very dry (we are considered as being in a Severe Drought zone). However, field trials should be conducted in other areas before using a material similar to Attapulgite. Attapulgite is marketed by Agsorb from Ochlocknee, GA and according to their website is a Attapulgite mineral in the non-swelling bentonite class. It is commonly called Fuller's earth. It's low bulk density and high absorbtivity allows higher liquid holding capacity. Our mineral processing maximizes the granule's micropore space. Agsorb heat treatments impart a hard inert granule with a high resistance to attrition.
Remember - you must work out your own blends! Clay like bentonite or attapulgite can present different issues if used without proper design and research.
We suppose some of the problems could be compatibility with roofing materials, clogging of drains and root rot when used in incorrect blend mixtures.
However in hot and dry areas where every drop (I am very serious about the 'every drop' statement) of water is precious on a green roof, these clays may really help with plant survival.
Sure, clay is not a substitute for proper plant design. Build you wind break and perimeter with CAM plants interwoven with appropriate Poaceae and C4 type photosynthesizing plant species.
But when you are at wits end (like we stay here in Jacksonville) about where the next drop of moisture will come from and how to preserve the liquid life, then this material just may offer help.
Watch your pH, watch your drainage, weights and water retention characteristics. Make sure you always study the MSDS issues.
Success will be found in the appropriate blend. Try starting with a 1-2% by weight blend, then adjust according to the site's wind and light exposure as well as temperatures.
We have had super great luck with these types of clays. As an added benefit many contain an amazing array of trace nutrients our plants require.
For a very interesting read on soil science, and the area from which we first started thinking about using clays see this very interesting article about Bonzai Soils.
And there embedded in linked article was ou answer to the high cost of SAPs or water retention crystals - and the alternative was a natural product, readily available (though mined :( - however it would behoove us to study the carbon footprint of SAPs v clays, including environmental impacts).
Oil Dry! Kitty litter!
We found a great deal at Sams Club here in the states - 40 pound bags of oil dry for several dollars.
Hey! Our green roofs are suffering in the hot and dry!
Good luck with your experimentation. You may find the effort was well worth your time. And your green roof may be able to stay greener longer!