Understanding the carbohydrate dynamics of sprouting Cirsium arvense (L.) Scop. and Sonchus arvensis L. ramets can assist in optimizing perennial weed management. However, detailed knowledge about general reserve dynamics, minimum values in reserves (compensation point) and different reserve determination methods remains sparse. We present novel insights into reserve dynamics, which are especially lacking for S. arvensis. We uniquely compare root weight changes as a proxy for carbohydrates with direct carbohydrate concentration measurements using high-performance liquid chromatography (HPLC). In a greenhouse study, ramets of two sizes (20 and 10 cm) were planted in pots. Subsequent creeping roots of sprouted plants were destructively harvested and analyzed for carbohydrates 12 times between planting and flowering. Efficiency in storing carbohydrates and the replenishing rate of root weight and carbohydrates was much higher in S. arvensis than in C. arvense. Thus, our study urges to evaluate perennial weed species individually when investigating root reserves. Determining root reserves by either using root weight changes as a proxy for carbohydrates or directly measuring carbohydrate concentrations by HPLC differed in the minimum values of reserves referred to as compensation points. For both species, these minimum values occurred earlier based on root weight than based on carbohydrate concentrations. Cutting ramets into 20 or 10 cm sizes did not significantly affect carbohydrate concentration or root weight changes for both species. We conclude that any practical applications targeting perennial weeds by fragmenting roots into small ramets through belowground mechanical control must be evaluated for trade-offs in soil structure, soil erosion, and energy consumption.

Management of perennial weeds has become increasingly difficult with the reduction of herbicide use. Creeping perennials accumulate reserves in specialized belowground organs from which they regenerate new plants after a disturbance. Through tool selection, tillage operations could be optimized to reduce perennial-weed reserves and limit regeneration. In the present study, the effect of five tools on the fragmentation of the creeping roots of Cirsium arvense (L.) Scop. (Canada thistle), a major perennial weed in arable crops, were analysed. A field trial was set up to measure the lengths of the root fragments left after tillage. Five tools were tested: mouldboard ploughing, rotary harrow, disc harrow, rigid-tine cultivator and goose-foot cultivator. Fragment-length distribution varied according to the tool: rotary harrow left the smallest (3.7 cm on average) and least variable fragment lengths, mouldboard ploughing the longest (12.7 cm) and most variable ones. The other tools produced intermediate-sized fragments (8-10 cm). Based on these results and literature, a model was proposed to predict perennial-weed regeneration probability from storage-organ fragments after one tillage run. The effects of six factors, which were agronomic (tillage tool), environmental (soil conditions and temperature) and biological (storage-organ fragment diameter, maximal belowground-shoot length and pre-tillage storage-organ distribution), were tested through a sensitivity analysis. According to the model, the probability of fragment regeneration success is lower for the rotary harrow than for the mouldboard plough. The most important drivers of fragment regeneration success were the biological traits: fragment diameter and maximal belowground-shoot length per unit fragment biomass. The present model should be complemented to predict the effect of tillage on perennial-weed regrowth and help improving non-chemical weed-management strategies. To achieve this, further research is needed on plant regrowth potential from storage organs and their architecture in the soil.

Fast regrowth from deep roots and rhizomes makes it difficult to mechanically control the perennials Cirsium arvense and Tussilago farfara respectively. It is, however, not clear whether new shoots originate mainly from fragments of roots/rhizomes in upper soil layers or from an intact system below depth of soil cultivation. Here we present results from three experiments with natural infestations of C. arvense, and two with both C. arvense and T. farfara. Plots of 1 m(2) were excavated to different depths (13-25 cm), all below-ground plant parts in the topsoil were collected and thereafter fragments were either returned to or removed from the plots. Regrowth from disturbed plots with removed or returned fragments was compared. The origin of regrown shoots, that is, whether they originated from seeds, intact below-ground root/rhizome systems or returned fragments, was examined. More C. arvense shoots originated from the intact root system (48%-84%) than from root fragments (16%-52%). The final aboveground biomass was not affected by removal of the top-soil fragments. For T. farfara, a small proportion (3%) of new shoots originated from the intact rhizome system, and the rest from fragments. We conclude that the intact root system of C. arvense contributes at least as much as root fragments to regrowth after soil cultivation, which might imply that time of treatment and depth of cultivation are crucial for the effect of mechanical control. For T. farfara, the results suggest that tillage equipment with high capacity to fragment the rhizome system will contribute to efficient control.

In the Northern Great Plains, organic production is often limited by creeping perennial weeds. Producers have typically relied on alfalfa to control these weeds but this is not always practical. Therefore, we investigated Canada thistle, field bindweed, and perennial sowthistle suppression from three diversified crop sequences: (i) ALF, three years of alfalfa followed by a fourth year of hard red spring wheat (HRSW); (ii) LENCL, lentil the first year, HRSW interseeded with yellow sweetclover the second year, yellow sweetclover green manure the third year, and HRSW the fourth year; and (iii) CCPLY, nine species cool-season cover crop (CC) polyculture the first year, HRSW the second year, nine-species cool and warm season CC the third year, and HRSW the fourth year. From 2019 to 2021, at peak weed emergence, Canada thistle density declined in ALF, but remained unchanged in LENCL and CCPLY. During peak weed vegetative growth, Canada thistle density in ALF was greatest during 2020 (7 shoots per m-2), then declined during 2021 and 2022 (1 and 0.6 shoots m-2, respectively). Within LENCL, Canada thistle density did not change from 2019 to 2020 (15 and 14 shoots m-2, respectively), but then declined to 0.3 shoots m-2 in 2021 before rebounding again to 5 shoots m-2 in 2022. Within CCPLY, Canada thistle density did not change over time (mean = 9 shoots m-2). Canada thistle biomass at peak weed vegetative growth followed similar patterns. Likewise, ALF was shown to suppress perennial sowthistle and field bindweed more effectively than LENCL or CCPLY. Hence, alfalfa remains a good choice for creeping perennial weed management. However, none of these weed species increased substantially in LENCL and CCPLY, and the final year HRSW yield did not differ among cropping sequences. Therefore, these two sequences may provide crop diversification options along with creeping perennial weed management.