Urban Ecosystems Research Group Blog University of Helsinki
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Yellow rattle (Rhinanthus minor) is a hemiparasite that steals water and nutrients from its hosts. This is done via haustoria that it grows around and into the host's roots (Figure 1). Yellow rattle can parasitize numerous plant species, but it favors grasses and legumes as hosts. Since these are exactly those that we wish to reduce when meadowing lawns, it might be useful to add yellow rattle in the mix.
Figure 1. Flowering yellow rattle and a haustoria (yellowish structure) of a rattle root (wide light brown) growing into grass roots (narrow dark brown).
The impacts of yellow rattle have been examined in a few studies in the British Islands and Central Europe on former agricultural fields or pastures and on seminatural grasslands. These studies have found promising, although rather varying, results. The addition of yellow rattle has decreased the biomass (or cover) of grasses, and sometimes also that of legumes, while impacts on other forbs have varied from positive to negative. Since it often reduces the most abundant species, it may help to create free patches for other species. The observed impacts on plant species richness and diversity have been positive or neutral.
The Lammi experiment seems to be the first examining the use of yellow rattle on former lawns. In this experiment each block received four topsoil treatments and half of the area was sown with yellow rattle. It is therefore possible to explore the impacts of the topsoil treatments on the success of yellow rattle, and also the effects of the rattle on other plants.
After midsummer this year, which was the midway of the first growing season, there was a field course on ecological research, and I got a group of students to supervise (Figure 2). The students explored the central 1 m2 of each experiment plot, and after only two days they had counted 9 600 yellow rattle individuals (out of which 3 100 were flowering) as well as numerous inflorescences of grasses and white clovers. The also estimated the cover of each of these (i.e. how many percent of the area the plant covers when looking from above). Meanwhile, I hurried through the same plots to make my own estimates of these covers and a few others that shall be later used to produce a scientific article.
Figure 2. Fantastic field course students and the beautiful cover of their report.
The students’ course report was finished in three weeks (unlike my article) and both the report and the yellow rattle proved to be very successful. It turns out that yellow rattle germinated, grew and flowered especially well on those plots where the soil had been left untreated (Figure 3). This was surprising as earlier studies have reported that light breaking of the soil increases the germination, survival and growth (by reducing competition). Thus, earlier studies have reported positive effects of soil scarification on the rattle, but in our case the scarified plots hosted slightly lower rattle cover and number of individuals (Figure 3). The stronger topsoil treatments (turnover and replacement) hosted very few successful rattles, so it seems pointless to add yellow rattle if such treatments are done.
Figure 3. The plots of one block, photographed from above. The plots that were sown with rattle are in the upper row. The photos were taken at midsummer 2023, which was less than a year after the treatments.
This year May and June were unusually hot and dry, which resulted in the death of most of those rattle seedlings that did not have other plants near to provide shelter (aboveground) or resources to steal (belowground). Especially the dry sandy meadow substrate (replacement material) was dotted with numerous dead seedlings at the end of May. A moister spring could have provided slightly different results. For example, perhaps scarifying would have been better compared to no treatment, which would have resulted in heavy competition from fast-growing plants.
But did the addition of rattle give the benefits that we hoped for? Yes, in terms of reducing grasses it was surprisingly efficient: Untreated plots with rattle had an average grass cover of 16%, while the untreated plots without rattle had an average cover of 42%. The number of grass inflorescences also decreased (average of 39 vs. 97 per 1 m2), indicating that the parasitism by rattle decreased the reproductive success of grasses. We also found weak evidence of decreasing the cover of white clover, but this effect was not significant as white clover was sparse on many plots even without rattle.
Although the beginning seems promising for the use of yellow rattle, we don’t yet know if it will have any long-term effects on the vegetation. The perennial meadow species that were sown last year are still too small and few to inspect (they were reduced by the drought of the early summer). Thus, we have to wait at least until next summer to see if the parasitism by the rattles has impacted so that these perennials have benefited from freed space, or perhaps if the rattles have ended up parasitizing some of these species too.
Anna Oldén
PhD, Post-doctoral researcher