Yellow Rattle is a wild flower that has for a long time acquired almost a cult status as the plant to introduce to a wild flower meadow to suppress grasses and so allow the wild flowers that grow with them, to flourish.
Yellow rattle is an annual found naturally in the wild in damp, undisturbed meadows where it is widespread but never dominant. Yellow petals protrude from the base of green bracts and as the flowers fade the bracts turn brown and encircle the seeds which rattle inside, hence its name. Bees feed from its flowers and one moth, the grass rivulet, eats the developing seeds in the pod.
It works to suppress rank grasses because it is semi-parasitic. Its roots penetrate the roots of nearby grasses and extract nutrients from them and reduce their growth by up to 40%. However, it also has leaves for photosynthesis, so it is not totally dependent on the host plant for survival.
Unfortunately, it is a mistake to think that yellow rattle only parasitises grasses. A total of plants from 18 other plant families are attacked with the legume family particularly susceptible, including clovers and trefoils. If the intention is to encourage the common blue butterfly, whose larvae feed on bird`s-foot trefoil, then it would be best to avoid introducing yellow rattle.
There is even a variation in susceptibility with grass species. Fescues and crested dog’s-tail grass are particularly susceptible but these are not vigorous, invasive grasses in any case and are ones to be encouraged. Whereas, more vigorous grasses such as ryegrass and couch are more resistant.
Some wild flowers respond to its attack by creating a lignifying barrier in the outer layers of the roots. Examples are ribwort plantain, ox-eye daisy and black knapweed. Consequently, these can grow well in yellow rattle meadows, sometimes at the expense of other wild flowers.
When it works, sowing yellow rattle works well, but I have seen meadows that have been made too thin from too much introduced yellow rattle. An attractive meadow will have both sufficient flowers for pollinators, and also plenty of leaves for larval and adult insects to eat. The grasses should not be too tall to overpower the wild flowers, but at the same time be thick enough at the base to give shelter from the weather and from predators.
I enjoy standing in a meadow on a sunny day watching the butterflies floating about, then a cloud shower passes over and they miraculously disappear until the rain has gone, having slotted their narrow bodies into the thick sward. Or grasshoppers jumping out of the way of my feet but when I dart down to catch one they also have disappeared. Many of the brown species of butterflies have larvae that feed on grasses along with 33 species of grass moths. The adult moths, at only1cm long, are adept at folding their wings around their bodies to hide or to work their way down to the base of the grass to avoid being eaten.
Farmers, in the past, regarded it is a weed, reducing their hay crop, and referred to it as Hay Rattle. In conservation it cannot be regarded as a problem in the same way but it should be used with care so it does not become too numerous. This should not be a problem because, as an annual, a simple way to stop it spreading would be cut the flower heads off after flowering. There will never be too much of a problem with a seed bank from it in the ground, as the seeds are only viable for 6 months. I would never think of it as a weed but just imagine if it was a perennial with a seed viability of 80 years like docks, then there would be a problem!
Mick Street
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Interesting detail here that I haven’t seen before that would be useful when thinking of introducing it to a garden – seed viability, what species susceptible or not, etc. Especially interesting to me was the susceptibility of birdsfoot trefoil which I have introduced to my lawn ( along with white clover ) and now forms some large patches in my lawn – so maybe not a good idea for me.
I think you are correct Ian. Bird`s foot trefoil and white clover are both low growing and survive well in a lawn that has to be cut rather than left as a meadow all season. They are good for pollinators and being legumes are also nitrogen fixing and give some nitrogen back to the grasses to help with the losses when the arisings are removed. Another good choice is self heal (not a legume). If you have mostly shorter grasses such as fescues, bents, meadow grasses and crested dogstail and you have to cut more than once a year then yellow rattle would not help. It works best where the meadow is left uncut all year until one cut in September and where the grasses really dominate and are taller species especially false oat grass. It can be interesting to leave a small area uncut all year and compare plant species and the different wildlife. A slow worm turned up in the long grass for me!