The one weed we get asked about the most, often in the winter after the season is finished, is perennial sowthistle. It has always been around, but seems to be increasing in number.
It is seen this time of year in patches. Later in the summer you will see groups of pretty yellow flowers above the soybean crops. It is a perennial weed that grows from root reserves, which makes it a tough critter to deal with. Glyphosate in the spring is not effective against sowthistle, which partly explains why this weed is increasing, because the root reserves allow it to regrow. Glyphosate will burn the top, but will not go to the roots, so the sowthistle grows back.
There are three strategies for controlling perennial sowthistle.
1. In glyphosate resistant corn, apply glyphosate as late as possible. Sowthistle will start to replenish it's root reserves when it starts to flower. A late spray can get to some of the roots and weaken the weed.
2. Apply Classic as late as possible in soybeans. Classic rates a 7 for sowthistle control in Pub 75, which means it is good, but not perfect. In glyphosate resistant soybeans, Guardian (glyphosate plus Classic) improves the score to an 8. The Classic needs to be applied before soybeans start to flower in late June.
3. Apply glyphosate in the fall, early September after wheat is best. This works because after sowthistle flowers, it will be replenishing root reserves and glyphosate will then translocate to the roots. We need good moisture for this to work. If the sowthistle is under drought stress we get lousy uptake of the glyphosate and the weed survives to come back in the spring.
Walk your fields now to locate patches of perennial sowthistle and plan accordingly.