SummaryThe success of weed management aimed at depleting the regenerative structures of perennial weeds depends largely on the sprouting activity of rhizome and root buds. Seasonal variation in sprouting of these buds on Cirsium arvense, Sonchus arvensis and Elymus repens was studied for plants collected from Denmark, Finland, Norway and Sweden. At 2-week intervals from July to October, 5-cm fragments of roots or rhizomes were cut from plants grown in buckets and planted into soil in pots, half of which were placed immediately into growth chambers at 18°C for 4 weeks. The other half of the pots were initially placed in a dark room at 2°C for 4 weeks before being transferred to the same growth chamber, also for 4 weeks. During the growth chamber period, the numbers of emerged shoots in each pot were counted weekly. The sprouting activity of C. arvense and E. repens was relatively uniform during this period and bud dormancy was not apparent. In all ecotypes of S. arvensis, innate bud dormancy developed during the latter part of the growing season. For all three species, differences in sprouting readiness were found among ecotypes. The results imply that C. arvense and E. repens are more likely to be controlled by mechanical measures in autumn than S. arvensis.

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Autumn growth of weeds (i) provides an opportunity for mechanical and chemical control in autumn and (ii) can be important for weed survival and infestation in the following spring. Growth characteristics of Elytrigia repens, Cirsium arvense and Sonchus arvensis in autumn were studied in 2004 and 2005, on plants of different origins and developmental stages (planted at various times from May to August). The plants were grown outdoors in large pots and were assessed during September and October each year. The study showed that (i) all species grew in autumn, but growth ceased and the species withered at different times; S. arvensis first, followed by C. arvense and then E. repens and (ii) less developed (i.e. younger) plants grew later in the autumn. This was demonstrated by leaf area development and biomass distribution during autumn. Older plants had a greater total biomass with relatively more rhizomes or creeping roots than younger plants. In young plants of C. arvense and S. arvensis, the biomass of creeping roots increased during autumn. The total biomass, however, changed little during autumn. These growth patterns indicate that E. repens will be the easiest, S. arvensis the most difficult and C. arvense in between, regarding control of these species in autumn.

Plants of catchweed bedstraw from different Norwegian locations and from three other countries were compared with respect to morphological factors, herbicide sensitivity, and genetic variation. For the morphological comparison of cotyledons, whorls, leaves, and fruits five populations, grown outdoors but sheltered from rain, were used. Plants from Belgium and Sweden showed a high similarity, whereas one Norwegian population differed significantly in nearly all parameters. The same populations were used for a comparison of the sensitivity to the herbicide mecoprop-P. In this study, only slight differences appeared between the five populations. Finally, a deoxyribonucleic acid (DNA) sequence analysis of the internal transcribed spacer (ITS) regions was performed. The entire sequence of the ITS1 and ITS2 and the 5.8S subunit of ribosomal DNA were obtained from 15 populations (12 from Norway and one each from Sweden, Belgium, and Germany). The sequences had a length between 590 and 662 base pairs; intraspecific length variation was observed. Based on six insertions–deletions and 26 nucleotide substitutions, two DNA types could be distinguished. The first type consisted of eight Norwegian populations, whereas the second one contained the other seven populations including all non-Norwegian populations. The sequence alignments were used to build a phylogenetic tree. The results of the morphological comparison mostly corresponded with the results of the ITS sequence analysis. The variation was only to some extent correlated with the geographic distribution of the populations.

A requirement for developing cover crop systems is to find cover crop species and cultivars that are both suitable and adapted in such systems, including sufficient frost resistance, for the local climate. The objective of this study was to detect the freezing resistance of different winter annual and biennial legumes, and to clarify whether the freezing resistance is correlated to the developmental stages of the plants. The study included a growth chamber experiment with nine species‐cultivars, including white clover (Trifolium repens L., cv. Milkanova) as a control plant, and three developmental stages. At different developmental stages the test plants were exposed to five different freezing temperatures. The best freezing resistance, independent of sowing time, was shown by hairy vetch (Vicia villosa Roth. cv.Welta). Crimson clover (T. incarnatum L. cv. Heusers Ostsaat) and yellow sweetclover [Melilotus officinalis (L.) Pall. cv. Nordgold Yellow] also showed good freezing resistance. Black medic (Medicago lupulina Gaertn. cv.Virgo Pajberg) showed medium freezing resistance, a little better than subclover (T. subterraneum L. cv. Denmark). Poorest freezing resistance was exhibited by barrel medic (M. trunculata Gaertn. cv. Parabinga) and snail medic [M. scuttelata (L.) Mill. cv. Kelson]. In hairy vetch, especially cv. AU EarlyCover, negative correlation between freezing resistance and age of the plants was detected. Generally, this was also true for crimson clover, subclover, barrel, and snail medic. In contrast, yellow sweetclover and white clover showed tendencies to positive correlation between freezing resistance and age of the plants. The results revealed the importance of developmental stage, at least in some species‐cultivars, in the determination of freezing resistance.