How to control weeds on a green roof
Spring has sprung and all over the UK plants of all shapes and sizes are growing fast. In the garden and on living roofs, some plants are more welcome than others.
What is a weed?
A weed is any plant growing in the wrong place. It might be grass between patio slabs, groundsel in the greenhouse or moss in the lawn.
Weeds that are commonly found on green roofs include grass, moss and tree seedlings.
Moss growing on a sedum roof – the moss is thriving at the expense of the more desirable sedums
Grass and broad-leaved weeds on a sedum roof
Tree saplings on a green roof – these have been allowed to get out of hand
The pretty white meadow saxifrage can be mistaken for a weed in the early stages of its growth. But as soon as the flowers appear it’s obvious that this particular plant is not to be weeded out.
How to treat weeds on a green roof
1. Control weeds on a green roof – know thine enemy;
Identify the weed if you can and try to work out why it is thriving on your roof.
Moss loves damp shade. Is there a problem with drainage on the roof? Or have we just had such a wet winter so avoiding damp soil has been impossible?
If your green roof is in shade for more than half the day then moss is almost inevitable.
So on a green roof with a deep substrate, try removing as much moss as possible. Then replant with shade loving plants like lesser Celandine, Vinca Minor or Wood Anenome.
On a shallow substrate roof, you may have to love moss. In the same way, if the roof can cope with the extra weight, increase the width of the pebble border.
Maybe, you can try to establish sedum cuttings in the shady area, but be prepared to do this at least every 2 years. Moreover, Sedum album is the most shade tolerant of the succulents that can survive on a shallow substrate.
Grass. Easy to identify and a regular visitor to green roofs. Wet weather and damp conditions will encourage grass to grow. Particularly here in the UK where our climate grows the best grass in the world.
Control weeds on a green roof – Grass is good…sort of
So grass is good …. in the right place. Not only that but native grasses are food for some butterflies and moths. The extra biodiversity is a boon if you can cope. Similarly, if your green roof has been properly installed the grass roots won’t damage your roof.
But if you hate the extra vegetation its weeding I’m afraid. On a shallow substrate roof you’ll often find that a period of drought will kill off the grass plants. Unfortunately, they simply cannot tolerate dry conditions as well as sedums can.
Tree seedlings are unacceptable on any green roof. Remove them ASAP, preferably by hand weeding. Because , if they’re allowed to grow, the roots may be strong enough to damage your waterproofing.
2. Take timely action to remove weeds
This applies especially to tree seedlings. Never let them get established.
3. Feed your living roof
With the weeds out of the way, encourage the plants you do want to fill in any bare areas. Applying a proprietary green roof feed will stimulate healthy growth.
A green roof feed will supply the nutrients that green roof plants need to produce a good show of flowers.
Feeding your green roof is an essential job for Spring and should be done every year. Please always follow the manufacturers instructions.
Getting help with a green roof weed problem
Should you need help or advice on green roof weeds, please don’t hesitate to contact us.
So all you need to do is email us with photographs of the whole roof and close ups of the problem areas and we’ll see what we can do.