In organic arboriculture, weed control is expensive. Groundcovers can be planted to repress weeds. Can this green groundcover contribute to the nitrogen supply or does it compete with the tree crop? An article in the arboriculture journal De Boomkwekerij reported on tests of green groundcovers conducted by Plant Research International of Wageningen UR. The entire research design and results have been published in a research report.
For suppressing weeds, green groundcovers are an attractive option. However, the method has not yet been used effectively in practice. During relatively dry years, the tree crop experiences too much competition from the groundcover and consequently falls behind in growth.
If growers still want to use a green groundcover, then white clover appears to be the most satisfactory choice. The clover should be mowed at the beginning of the second growing season, at the middle of April or beginning of May, depending on the spring weather. This not only reduces the competition for water, but nitrogen is also released from the crop residue. For good tree growth, however, additional irrigation must still be used if necessary .
Three cover crops – oil radish, rye and summer barley – were sown in mid-August between rows of the tree and hedge species Carpinus betulus, Corylus avellana and Ligustrum vulgare. With high levels of available nitrogen, the groundcovers contributed little to reducing nitrogen leaching. Because the groundcovers became established slowly and with difficulty, their capacity to suppress weeds was also limited.
White clover, sown full-field in the autumn on a trial field with first-year fruit trees, provided excellent weed suppression during the second growing season. These fruit trees grew well in total length, but the length of the branches remained somewhat below standard. After being lifted, however, the fruit trees grown with clover as an groundcover had more well developed and active above-ground adventitious roots. This is undesirable because these adventitious roots can become infested by pests and infected by diseases such as fruit tree canker.
It is also possible to sow clover immediately after planting the tree crop on a weed-free parcel. In 2008 and 2009 the clover varieties Alice and Barbian, sown after planting, kept the soil sufficiently free of weeds, but still competed with the fruit trees for water and nitrogen. In June 2009, the amount of available nitrogen was shown to be too low. Ultimately, 20% of the fruit trees that were grown with clover groundcover complied with the highest quality standards, compared to 47% of the trees grown on bare soil; this was primarily because the length of the branches was shorter in the crop with the clover groundcover.