Invasive common milkweed (Asclepias syriaca L.) and sugar beet.
The common milkweed (Asclepias syriaca) is an invasive alien species of European Union concern, capable to reproduce vegetatively and generatively in our geographical conditions. From an agronomic point of view, the common milkweed is dicotyledonous, deep-rooted perennial, weed and is still one of the difficult-to-control weeds in cropping system. Common milkweed seedlings become perennial within 21 days after germination because plants capable of producing new shoots from buds and plants grown from seeds seldom flower until their second summer. The common milkweed is very easily recognizable and cannot be confused with any other plant. It is desirable to use this specificity in its initial, however unique, occurrence on a plot. The most effective control is to prevent the occurrence on a new site, which can be achieved by avoiding seed formation on plants of already existing colonies on neighbouring plots. Sugar beet as a row crop creates an ideal environment for seedling growth, germination and seed emergence, mainly due to its slower growth in the early growth stages. Most milkweed seedlings can be controlled in the first days of their life, within 21 days of emergence from the seed by digging, shallow cultivation (5 cm) or herbicides. In older plants, soil cultivation is considered less appropriate because of the production and spread of viable roots fragments. From the viewpoint of herbicidal protection, it is known to date, that the milkweed is sensitive to only one active substance authorized for use in sugar beet, to the controversial glyphosate. Greater accumulation of glyphosate in the dormant root buds can be achieved by addition of synthetic cytokine - 6-benzylaminopurine (BAP) 3 days before glyphosate application. Dormant root buds are stimulated and subsequently killed by glyphosate at the dose 1.1 kg ha-1. Potential efficacy of other active substances authorized for use in sugar beet needs to be verified, or other active substances will need to be authorized by verifying their applicability in sugar beet stands. Effective surfactants are those with a high hydrophile-lipophile balance (19-20) and cationic character. The use of surfactant is necessary to enhance the herbicidal toxicity.