Weeds on green roofs

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 also 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. 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 that avoiding damp soil has been impossible to do?  

If your green roof is in shade for more than half the day then moss is almost inevitable. 

On a green roof with a deep substrate, try removing as much moss as possible and replanting with shade loving plants such as lesser celandine, vinca minor or wood anenome.

On a shallow substrate roof, you may have to learn to love moss. Alternatively, if the roof can cope with the extra weight, increase the width of the pebble border.  

You can try to establish sedum cuttings in the shady area, but be prepared to do this at least every 2 years.  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 whole world.

Grass is good …. in the right place.  Native grasses are Laval food plants for some butterflies and moths so if you can live with the extra biodiversity please do so.  If your green roof has been properly installed the grass roots won’t damage your roof.

If you’re hating the extra vegetation it can be weeded out by hand. On a shallow substrate roof you’ll often find that a period of drought will kill off the grass plants.  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. If 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 without fail.  Always follow the manufacturers instructions. 

Getting help with a green roof weed problem

Should you need help or advice on green roof weeds, the green roof maintenance team have a wealth of knowledge and experience.  

All you need to do is email Kevin Docherty with photographs of the whole roof and close ups of the problem areas and he will be happy to advise.

If you need help with green roof maintenance, talk to Enviromat.  After a relaxed chat, you can help decide what work needs to be done, outline any possible problems with access and agree a date and a price for the work. It’s a straight forward process and the people at Enviromat have all the right tools and training to be able to do the job properly.