Interactions between root hemiparasitic plants and their hosts are strongly affected by host identity but may also depend on the condition of the host. An important determinant of host quality could be host age, as it may influence host size, allocation patterns, responses to infection, and the strength of competition for light between parasite and host. We investigated the effects of host species identity, host age and above-ground separation of hemiparasite and host on the interactions between the hemiparasite Rhinanthus alectorolophus and five host species in a factorial experiment. The host species were planted at six different times, from ten weeks before the parasite was planted to four weeks after. Host age strongly influenced the performance of the parasite, but these effects also varied among host species. Parasites grew largest with hosts planted at the same time or two weeks earlier, but their performance strongly declined both with increasing host age and with the time they grew autotrophically. A large part of the variation due to host age but not of that due to host species identity could be related to the negative influence of host size at the likely time of parasite attachment. The low quality of older hosts was not due to light competition, suggesting that effective exploitation of these hosts was prevented by other factors like harder roots, stronger defence against parasite attack or competition for resources taken up by the host roots. Suppression of host growth by the parasites declined with increasing host age. The results indicate that the choice of host age may influence the results of studies on hemiparasites. They also highlight the importance for annual root hemiparasites of attachment in early spring, i.e. at a time when their mostly perennial hosts produce fresh roots but are still poorly developed above ground.

The dataset is a collection of plant traits from plants grown in a pot experiment in 2 growth chambers. We grew the root hemiparasitic plant Rhinanthus alectorolophus with several host plants. We used five host species and six different host ages (hosts were planted 10 weeks before the parasite, 4 weeks before the parasite, 2 weeks before the parasite, at the same time as the parasite, 2 weeks after the parasite or 4 weeks after the parasite). In half of the pots, above-ground competition between hemiparasite and hosts was prevented by separating hemiparasite and hosts by an aluminium foil. We also grew hosts without parasites with all possible combinations of host species and age and we also grew parasites without hosts. Several traits of hosts and parasites were measured at the end of the experiment (9 weeks after parasite planting). 

To study potential influences of host traits at the time of parasite attachment, we also grew hosts without parasites and measured them at a time when the parasites had grown for two weeks in the other pots.

Analyses were carried out with R version 4.2.1. 

Microsoft Excel (version 2007 and newer) is the recommended program to open and work with the data (format .xlsx), but WPS Office Spreadsheets, OpenOffice Calc, LibreOffice Calc and Apple Numbers could also be used to open the file.