Spring 2016 was one of the coolest and wettest in my recent memory which is great news for plants, but may have provided challenges for green roof managers.
1: Difficult access has delayed maintenance visits
Health and safety is the number one concern when working on a roof. Common sense tells us that it’s not a good idea to be standing outside, on a roof when it’s blowing a gale and the rain is sheeting down. Which means, that maybe you’ve not had a chance to get on with your spring time maintenance yet.
It’s important that excess rainwater can be channelled off the roof as quickly as possible. This will add extra weight to the green roof build-up. In most cases, there’s no need to panic, the architect who designed the roof ought to have accounted for a little bit of extra weight for short period of times – the technical term for this is “live loading”.
What’s more worrying about having a saturated growing medium is that the plants will be stressed. Water displaces air from the spaces between the particles of substrate and without air, the plants’ roots will drown and the plants will perish.
So, if you haven’t had a chance to clear out those drainage channels yet – please do it ASAP (but safely!)
You may not have been able to apply a feed either and that’s something that definitely should be addressed quickly. See point
2: Too much rain can affect nutrient levels
Plant nutrients are soluble in water. They have to be, otherwise the plants can’t absorb them through their roots.
You probably know from GCSE Science that green plants use sunlight, carbon dioxide and water to create their own energy food, but that doesn’t give them a balanced diet. They also need nitrogen (to make proteins for growth and for metabolism), Potassium (for hardiness and good health), Phosphorus (for flowers and reproduction) and a whole plethora of micro nutrients that are the equivalent of our vitamins and minerals. All of these things are absorbed from the growing medium via the roots.
If the correct nutrients can’t be found in the growing medium, the plants won’t be properly nourished and they won’t thrive.
Unfortunately, all the rain we’ve had this year may well have dissolved a high proportion of those essential nutrients and carried them out of the reach of the root systems.
If your green roof is in a high rainfall area and has a relatively shallow layer of growing medium, it will be well worth applying an extra feed in the coming weeks. Especially if more rain has been forecast. Make sure granular feed is watered in though – it can scorch the leaves if not.
3: Wet growing medium promotes the growth of weeds
Weeds! You have to admire their tenacity, they’ll take advantage of any situation. Extensive green roofs tend to be designed with an in-built weed control mechanism, however, this persistently wet weather will have neutralised this for now.
This sloping sedum roof has a few grass plants growing amongst the sedums. At the moment they don’t look too bad, however, if left unmanaged the grasses could take over and the pretty sedums would be lost.
Normally on an extensive green roof, the shallow layer of growing medium, combined with the use of drought tolerant plants, allows unwanted plants to be “droughted out” during the summer months. Something tells me that’s unlikely to happen this year and so the green roof managers need to keep on top of the situation.
Hand weeding is by far the best solution. It’s tedious and time consuming I know. But without it, you’ll find the plants you WANT to grow on your roof may be outcompeted by the ones that are less welcome. In time, that will mean a lot of hard work to return the roof to its ideal condition. As my Nan used to say, “A stitch in time saves nine”.