Weeds invading sedum green roof

by L

Can anyone help? We have a 5 acre green roof covered in five varieties of sedum. Last spring we added 24,000 additional sedum to fill in sparse areas. As we planted we added a slow release fertilizer. We have great looking sedum, but even better looking weeds. We've tried hand cultivation and only got more weeds this spring. Any suggestions? I hate the thought of using a chemical, but has anyone over sprayed sedum with any type of broadleaf control and not damaged the sedum plants?


Drought Smart Plants reply:

Weeds on a Sedum green roof are indeed a problem that doesn't get much attention. Going to all the work of installing such a huge roof only to be invaded by weeds that love the conditions just as much as Sedum is very disheartening.

I highly recommend the use of pasteurized or engineered green roof soil when you build your roof, as this hopefully won't contain any weed seeds. However, even the most pure and uncontaminated soil will eventually get some weed seeds germinating as they will inevitably be brought in by birds and the wind.

Please don't under any circumstances spray with any chemical - not only will you be completely going against the whole goal - which is to be more earth friendly - you will also most likely kill off the Sedum already established, as well as causing toxic run off next time it rains.

I know you won't want to hear this, but hand weeding is probably the only way you will get rid of the weeds, and it may take a couple of seasons to get them all and the Sedum grow big enough to compete enough to smother any more.

You don't say if you mulched with anything? (Probably not, on that size of roof)

What kind of weeds are they? - know your enemy!

What stage are they in? - perennial weeds usually make a rosette the first year, then bloom the second, at which stage they are easier to pull out.

Another option is burning them with a propane torch - make sure you don't accidentally burn the membrane and only do this if the soil layer is deep enough.

Good luck with your project - please keep me posted as to what works!

Jacki