Changes in the weed flora in Northern Hungary, after three to five years of set-aside

This is the first time that an extensive study covering several has been ca rried out in Northern Hungary on set-aside land. In ten districts of Northe rn Hungary, changes in the weed flora of uncultivated fields and plots were examined on dessignated areas tone per district) three times a year for fo ur consecutive years. For brevity's sake, this article includes only the re sults obtained for areas where cultivation had just been given up at the be ginning of the study. The most important findings are the following. The gr eatest changes in the stucture of the weed flora usually take place in the third or fourth year after cultivation is stopped. These changes may go in two directions: either a perennial weed becomes prominent, e.g. Elymus repe ns, and in this case the weed flora is less diverse, or the surface of the fallow becomes heterogeneous and patch-like. In the latter case the proport ion of such weeds as Picris hieracioides, Erigeron annuus, Daucus carota, C alamagrostis epigeios, Epilobium tetragonum, etc. increases. When re-cultiv ation is started, the situation is less favourable in the first case, while in the second the weeds are easily contolled even mechanically. Since spec ies are interchangeable, the decisive factor is not the presence of certain species, but the different life forms and the spread of certain aggressive perennial weeds. In the second case the direction of change is more favour able, because it is the less dangerous weeds that increase. On the basis of the four-year study, it can be stated that the effect of set-aside land on adjacent, cultivated areas is much less than was previously assumed, and i s often insignificant.