Drip Irrigation for Sedum based Greenroof

Maria,

Your instincts are correct. Brad Rowe of Michigan State University presented research findings from comparative testing done for watering efficiency and effectiveness between overhead irrigation, surface drip irrigation and sub surface irrigation at last fall's Green Roofs for Healthy Cities Conference in Chicago. The study found that on extensive green roofs (which most Sedum green roofs are), overhead irrigation is by far the most effective and efficient. Followed by surface drip irrigation. Sub surface irrigation came in dead last in both categories. Overhead irrigation is the most effective way of watering extensive green roofs. It just needs to be managed properly to prevent evaporation before the water enters the growing medium. This is done with pulse irrigation (soak & cycle) and appropriate timing of irrigation cycles as well as selection of appropriate irrigation heads (gear drive, low arc, large droplet, low pressure). Overhead irrigation is by far the most economical to install, operate and service.

I am not familiar enough with the climate in Peru to give you advice on which plants to use, but I would not be surprised if most of the Sedum that we use in the Great Lakes Region are not suitable for your climate. It is likely better to look for more tropical succulents and cacti to fill that niche.

Hi Maria,

Great to hear you have a green roof in the works! Just a note on the subsurface irrigation comment and the Brad Rowe study: Brad Rowe from Michigan State University was actually my professor while doing green roof research at MSU. I still keep in contact with him and actually met with him last week. He is a top notch professor and has done some of the best green roof research in the world! He did do a quick study on irrigation but a note on that study - it was done in a green house which does not take into account a rooftop environment. I talked with Brad about this study and he does admit there were variables which need to be taken into account when looking at the results some of which I outline below:

- The study was done in a Green House. It did not take into account evaporation which happens in overhead irrigation systems which would occur even more so on a roof top environment (wind).

- The study did not take into account how much water was input / used with overhead irrigation vs. subsurface. After 30 minutes of water being used overhead vs. sub surface: overhead will have more water out put and there fore would retain the greatest amount. So this is not a real assessment of overhead vs. sub surface irrigation.

- The study says “the moisture retention fabric was effective in retaining water but for sub-irrigation a visible water front was not visible as water did not reach the surface via capillary action. You do not actually want the water to reach the surface (surface irrigation/water encourages surface rooting, which is vulnerable to extreme temperatures and moisture stress. Weed seeds are less likely to germinate if the substrate surface is kept dry.) As long as water moves into the root zone (2”-3”) plant roots can access and use it. We have found water moves 3”+ into the root zone through capillary action. Over head irrigation promotes roots moving toward the surface to access water especially in a green roof environment where they're is only a set amount of soil depth.

- Capillary mat spreads water evenly over the entire area which in turn can irrigation a vast area of green roof efficiently. The study is correct in that drip lines are not efficient in irrigating a vast area of ground cover. However integration of drip lines + capillary / retention mats effectively spreads water evenly to the entire plant root zone.

Some problems with overhead irrigation include:

- Wind on a roof is a factor which makes overhead irrigation unsustainable.
- Overhead spray irrigation can be waste full in a roof environment and also encourage surface rooting, which is more vulnerable to extreme temperatures and moisture stress.
- Weed seeds are less likely to germinate if the substrate surface is kept drier.
- Buried pipes are more efficient than surface pipes. They do not see the sunlight therefore will not degrade.
- Overhead spray heads take maintenance to ensure they are working properly, pointing in the right direction...and also take more water pressure which equals more valves needed equaling higher costs.

I have been involved with studies at Michigan State which have shown that moisture and temperature correlation at the bottom of the green roof root zone is very important in the health of the green roof plants - Sedum in particular. This data will be published and I can send some along if you might be interested in seeing it. This would point toward the optimal method of irrigation being subsurfacely through - capillary mat / drip line connections. This subsurface irrigation method integrates a capillary retention mat into the design. Drip lines which sit on top of a capillary mat drip water onto the capillary mat which then spreads and holds water for the plant roots to access and use where they need/want the water - at the bottom of the root zone.
It is also the more sustainable, water efficient way to go.

If you still want to use overhead irrigation - you could design the system to include subsurface irrigation and overhead (if you really wanted overhead just incase). If you do this I'd recommend putting the subsurface on separate zones (valves) and the overhead on separate zones...this way you could irrigation the majority of time subsurfacely and as needed or if you wanted turn the overhead on sparingly so as to not waste larger amounts of water.

If designed and installed correctly subsurface drip irrigation does work very well and is the more water efficient way to go.
Those are just my thoughts and recommendations based on work in the green roof research industry as well as work on green roof system installations. I can send you some pictures of projects we have worked on which included subsurface irrigation / capillary mat systems which have been very successfull.

Good luck with your project! Its great to hear green roofs are moving in Peru!

Kind regards

Jeremiah Johnson