Help nature recover

So the flowers are out and the green roof maintenance season is in full swing.  The grasses are lush and the roofs are blooming. But here at Organic Roofs our new team of rooftop gurus have all been asking the same thing.  How can you tell the Asteraceae family apart?

How’s you say it: asterace-eh?

The Aster sisters, all with a wild yellow mane of hair (aka petals) and all looking the same to the untrained eye.  The Hawkbits, cousins of the Cornflower, daughters of the Dandelions, ancestors of the Sow Thistle, with ‘smooth’, ‘beaked’, ‘marsh’ and ‘rough’ varieties. Not to mention the questionably named Oxtounge twins and a very dodgy uncle named Nipplewort.

Why does it matter?  Well, some of this family will flower harmlessly, contribute equally to a beautiful diverse rooftop meadow, reflect the golden sun and produce delicious nectar for pollinators of all shapes and sizes.  Some however, will delve deep with their strong tap root, potentially trouble the waterproof layer and reproduce so effectively that before long you will have roof top of pure yellow followed by white fluff that will colonise any bare bit of soil at rooftop or ground level.  This could potentially make you very unpopular with your green fingered neighbours who diligently remove any rogue Dandelions each spring.  Although we all remember the usefulness of these Dandelion clocks for childhood time telling, they are not a species you want on a green roof!

So how to tell them apart?

The questions to ask to these gals to determine friend from foe are as follows.

Firstly, does she have one (flower) head or two? or more?  And if so, where are her heads on her body (stem)? Bunched together at the top, or hanging halfway up? Are the flowers solitary? one head per stem? or multiple flowers per stem? Are the stems branched or unbranched? Are the stems leafless or leafy? Finally, Bracts.  Bract what? you say . . . .

 Bracts are the part of the the bud.  They encase the flower before it bursts forth.  They are often found at the base of the flower where it meets the stem.  With this family they can be the key to distinguishing sister from sister and mister from mister. Check the bracts. how many rows are there? Are the rows different? Are the bracts shorter in the outside row?

A little investigation should determine who’s who.  Here is how an interview might go and the results of a brief encounter between myself and a flower on our patch of wildflower turf at the Organic Roofs headquarters.

“Hello beauty, who are you?”

[Flower Silence] 

“What is your name?”

 [Flower Silence]

 “Im awfully sorry i can’t understand you, I hope you don’t mind if i have a closer look.”

[Flower Silence]

I take that as a yes and  lean forward examining the specimen… Hmmm…

So only one head, solitary right at the top of a branched, hairless stem.  A few tiny leaflets running up the stem, so small they almost look like scales rather than leaves.

Solitary head on branched stems huh?  Are you a Hawkbit? But Hawkbits are hairy and your stems are smooth. You’re too tall to be a Hawkweed, and its too late in the year for you to be a Hawksbeard.

Lets check the bracts – a ha!  Alternating bracts almost feather like around the base of the flower, tipped with a dark colour on each one – undoubtably a Catsear. Hypochaeris radicata, common, native, perennial and harmless to waterproofing.

“Nice to meet you Catsear, my name is Dora, so pleased to make your acquaintance.”

[Flower Silence – although now i imagine this silence is friendlier!]

At the end of the day I leave the office and the sun has moved off that patch of ground. I notice the whole flower has closed as if snuggling up for the night.  Is it in reaction to the temperature drop?  The change in light?  Or has it just finished flowering and is dying back in order to start transforming into a seed?  The next day the flower is out again, strong in the morning sunshine, so it seems the Catsear likes to tuck itself in each night, just like its namesake.

If at all unsure with this varied and similar looking family, one sure way to be sure you have found a dandelion is to break the stem.  If it is hollow and a white sap come out of it then you have yourself a dandelion, best whip it out, tap root and all if its on a roof.  Although if its in a field, i say leave it – yellow makes me happy and how else will children know what time to head home for their tea?

Got a similar project? We'd love to help

Drop us a line at


or give us a call on

0203 828 7505