If you have a yard full of dandelions or creeping Charlie that you have been spraying all summer, you know that they are still laughing at you. There is a good reason for this. In the summer, while herbicides may damage a few leaves, it doesn’t kill the stinkers. Some herbicides are systemic, they move within the plant. In spring and summer, plants are moving sugars up from the underground storage parts to produce new growth. Herbicides don’t move against the flow very well. This upward movement continues until flowering. This means that less herbicides get to the roots. Perennial plants may be tough enough that even with this damage, they are able to regenerate. In the fall the reverse happens. The plants are storing sugars for the winter by drawing it down into the roots. This downward movement of sugars really kicks in the first three weeks of September. It continues as long as the plants aren’t killed by frost and 50-75% of their leaves are still green. That is why applying herbicide now is so much more effective in killing the stinkers.
Contact herbicides kill the leaves of the plant but don’t move into the roots so the weed will most probably grow back. Finale is one of these. Others may contain potassium salts of fatty acids. Horticulture vinegar and boiling hot water work in similar fashion.
If you are seeing a broad-leafed grass in your nice bluegrass lawn, you have crabgrass. This is an annual grass and the best you can do for it now is to keep it mowed so it doesn’t go to seed. You treat that next spring with a preemergent seed killer like corn gluten meal.
Some plants are biennial. They need two years to complete their life cycle. The first year they are just a clump of leaves and require a winter to flower and produce seeds the second year. They start from seeds so a preemergent seed killer will reduce them if applied at the proper time. Perennial weeds like dandelions shed lots of seeds after blooming. You can dig the “adults” if there aren’t too many, then apply the seed killer.
Most broad-leafed weeds like dandelions, are very susceptible to herbicides containing 2, 4-D. Charlie and violets usually can be controlled with herbicide containing triclopyr. Quackgrass needs Roundup to really kill it. Unfortunately, while Roundup kills the grass, it leaves the dead stuff standing so you will still need to either cut it or pull it out.
Don’t stop pulling annual weeds out of your gardens now. They will be going to seed and leaving you with millions of their babies next summer.
Start saving your leaves as they fall. Use them to mulch your flower beds. The experts demand that you wait until a hard frost to spread mulch. Usually, by that time at least ¾ of the leaves have fallen. So you get a few mice under the much. All they can do is eat the tops off plants. You will be cutting that off anyhow. Spread them when they fall and let the experts mulch in the first snowstorm. We will be in by the fire reading seed catalogs.
Bev Johnson is a Master Gardener with the University of Minnesota Extension. Her column appears in the Weekend Edition.