A living green roof potentially plays host to many different kinds of plant. Not all of them were originally planted on the roof. Not all of them are welcome.
A weed is any plant that is growing in the wrong place. If you have plants on your green roof that you don’t think ought to be there – you have a weed problem.
Common green roof weeds and how to treat them
Moss is generally found in damp, shady places. It’s not uncommon to find moss growing on a sedum roof where the sedums are failing.
Ecologically speaking, moss is a valuable plant. It may look different to what you had first planned but it’s unlikely to be affecting the functionality of the roof. However, if moss is present, it could be a sign that you have problems with your green roof.
First look at the reasons why this plant has established in the first place.
Has the weather been unseasonably wet? – The problem will normally fix itself once the weather changes. Remove as much of the moss as you can by hand (especially from around drainage outlets) and give the remaining plants a good feed to bolster them up.
Is there a problem with drainage? Check that all of the drainage outlets and the gutters are clear. If your green roof is wet for long periods of time there are very few plants – apart from moss – that will survive on it. Talk to a surveyor if you’re at all worried.
Is your roof in shade for long periods at a time? Careful plant choices will help to keep the moss at bay. Forget sedums or sempervivums, they need sunshine for at least half the day or they will not survive. If you have a nice deep layer of growing medium, replant the area with shade tolerant wild flowers. Bugle does well in shade and it has bee-friendly flowers.
Are the sedums failing due to lack of nutrients. This is the most likely scenario. Sedums are by no means greedy plants, but they do need some food. Especially in spring time. When they’re hungry, their leaves fail to change from red to green in spring time and the plants stay small. That leaves gaps between plants where moss can establish itself.
Feed your roof and the sedums will outgrow the moss.
Grass seeds get dropped onto the roof by birds and are sometimes blown there by the wind. Grass is unavoidable but it IS controllable.
Grass will thrive on wet roofs. Make sure that rainwater can get away fast by cleaning out your gutters and unblocking your drains.
Don’t let the sedum plants get so hungry as grass can sneak into the gaps between them. Be vigilant with your maintenance regime and make sure you apply a sedum feed every spring – without fail.
Try to pull up grass plants before they set seed and consider spot-treating them with weedkiller if they’re a real nuisance.
These are inevitable especially if you are near to Ash, Sycamore, Elder, Birch or Buddleia trees. Remove seedlings as soon as you see them. They have strong roots that are quite capable of damaging your waterproofing and that IS expensive.
These green roofs have been designed to cope with the demands of growing trees – the weight, the depth of growing medium and the irrigation. If your green roof was not designed for growing trees – don’t let seedlings get established!
How to avoid weeds on a green roof
Careful design, proper construction and sensible plant choices are the first steps to avoiding a weed problem. But the true secret is to never ever neglect your green roof maintenance. Just because it’s out of sight, it shouldn’t be out of mind. Write it on the calendar, set an alarm, do whatever you need to do but always feed your green roof in spring and keep those gutters clear.
Green roof design advice