Every Wednesday, I hope to find a new ‘weed’ to investigate. My only criterion will be that I will not have deliberately planted the subject of our inquiry. Who knows what we will find…..

Dear Readers, I found this plant carpeting the graves in some of the more open, sunlit areas of  St Pancras and Islington Cemetery when I visited with my friend Ann a few weeks ago. It is always a surprise to find a succulent growing in the wild: with their water-storing leaves, I associate them more with climates where moisture is hard to come by, something that can hardly be said of the UK. However, the soil is very scanty on some of these burial places, and the addition of gravel or small slates has made the area even more free-draining.

In fact, this plant, also known as Large Rock Stonecrop, is an ‘alien’, although it has been here for almost four hundred years. It was originally introduced as a salad plant, but was recorded in the wild for the first time in 1666, and has been making itself at home ever since. There is something very toothsome about the leaves of Sedum plants – they remind me of tiny asparagus stalks. Maybe one day I will summon up the courage to try them as food.

The vigorous growth of this plant, coupled with its hardiness and resistance to drought, is one reason why it is a key component of many green roofs. These have become really popular during the past few years, and they have a lot of advantages – they help to insulate the rooms below, provide an additional habitat for invertebrates and other creatures, can help to absorb rainwater and reduce storm run-off, and can help to cool the air in urban areas through taking up water and releasing it by transpiration.

There are several different kinds of Green Roofs. Extensive roofs can support 10-25 pounds of vegetation per square foot, whilst intensive roofs can support up to 150 pounds. Sedum is the usual planting on extensive green roofs. These are often the roofs of choice – the substrate for planting is not so thick (typically just a few centimetres, and some roofs even have the Sedum planted directly into Rockwool), and therefore the requirement to  reinforce the roof to carry the extra weight is not so onerous. Furthermore, these roofs are practically maintenance free – someone just needs to climb up once a year to replace any ailing plants, and to pull out any perennial ‘weeds’ which are in danger of penetrating the waterproof layer beneath the soil with their roots.

Now, I am all in favour of Green Roofs, and when I’m feeling particularly bossy (which is most of the time) I think that any new flat roofs in cities should be Green Roofs as a matter of course. In fact, the city of Toronto already mandates this for residential buildings over six storeys tall. Think of the difference all those Green Roofs could make on the 200 new skyscrapers that are planned for London! Fat chance of that, I suspect. But although I think that Sedum rupestre is a wonderful plant, I do think we could be more adventurous. Take a look at some of these:

Of course, nothing is new under the sun. Sod roofs have been used in cold climates for hundreds, if not thousands of years, and very fine they are too.

On their own, Sedum roofs tend to be much less biodiverse than roofs that include a greater mixture of substrates and plants (a study was done by G.Kadas in 2006 on just this question). But even these relatively cheap and easy-to-install roofs can have their attractiveness to wildlife increased by seeding them with appropriate (i.e. local) grasses and herbs. There is a wonderful free publication by BugLIfe here . One day, when I’ve saved up my pennies, I’m going to put a green roof onto my garden shed. Here in the city, we need to maximise every corner’s potential as useful habitat for the creatures around us. After all, we’ve stolen most of the space they need. It wouldn’t hurt us to give something back.

Well, after that digression, let’s get back to our patient little Sedum.

How do you know if you’re looking at Reflexed Stonecrop? Well, the leaves have a habit of turning backwards, hence one of the plant’s Latin names, Sedum reflexum.

Here, the flowers are shown with six petals, but according to my Wild Flower key, they sometimes have seven. It seems that the Sedum hasn’t read the book, and this is often the way with plants – just when you think you’ve worked out what species they are, and are ready to whack a label on them, they throw you a curve ball. And don’t get me started on hybrids.

On the Seedaholic website, the author mentions that another name for Sedum reflexum is ‘Love Links’. The author speculates that, because the plant contains phytoestrogens which reduce testosterone, it could have been used to reduce male aggressiveness, and to encourage marital harmony. Maybe a stonecrop could be sent to some of the more bellicose of our world leaders to see if it did the trick with them too?

Finally, there is a long-standing belief that if a Stonecrop which is being kept as a houseplant dies, this is a sign of impending domestic trouble. And I can well believe it. If things have got to so bad that this hardy, tolerant, undemanding little plant has been killed off, the household must indeed be in a sorry state.

Resources this week include ‘Nature in Towns and Cities’ by David Goode in the New Naturalist series. I cannot recommend this book highly enough. It’s certainly worth getting a copy from your local library (if you are lucky enough to still have such a thing).