Crops: The Lynch File - Why are perennial weeds still a problem in Ontario?
Wednesday, November 4, 2009
Though they are easier to kill than annuals, perennial weeds still cause problems because growers do not understand their biology or fail to kill them off completely
by PAT LYNCH
Perennial weeds are easier to kill than annual weeds. Give me a field of perennial sow thistle, quackgrass or bindweed any day instead of a field of ragweed or lambs' quarters. When controlling perennial weeds, you have to understand where they are coming from. Once you know this and their weaknesses, they are easy to control.
Perennial weeds generally reproduce from root sections. Once you kill this root portion, you can readily control any new plant coming from seed. Perennial weeds do produce seed, but these seeds are short-lived compared to annual weeds. They also have a low percentage of viable seeds. And, since perennial weeds coming from seed are not very aggressive, tillage and herbicides will control them. Obviously, all perennial weeds generally enter a field by seed but, once established, their main way of reproduction is through their extensive root system.
Annual weeds reproduce by seed. Lowly plants like lambs quarters or ragweed can produce hundreds of thousands of seeds per acre. Many of these seeds can lay dormant for years. Once an annual broadleaf weed goes to seed, it will be a problem in that field for a long time.
Annual weeds starting from seeds are aggressive and competitive. Even if a perennial weed goes to seed, the chances of that weed spreading by seed are much lower than annual weeds that go to seed. Have you ever seen a quackgrass plant that started from seed? I have not.
So how come perennial weeds are still a problem in Ontario? There are two main reasons. First, growers do not understand the biology of perennial weeds. The second is that growers do not completely kill off a perennial weed.
The biology of perennial weeds differs by species. But all have a vulnerable time in their life. Weeds like bindweed and milkweed are vulnerable when they are flowering.
Herbicides applied at that stage are translocated to the roots to start the kill.
Perennial sow thistle is also vulnerable at flowering, but more vulnerable in the fall when it puts out new rosettes. These rosettes move food to the roots for overwintering. Kill all those rosettes and you make a big dent in a perennial sow thistle stand. If you spray these rosettes, the chemical is moved to the roots to start the kill.
The perennial grass, quackgrass, is very vulnerable at the three-to-four leaf stage. The issue with quackgrass is that often not all of the shoots have three-to-four leaves at the same time. Some shoots will be dormant.
In all cases, you have to presume that your first attack on these weeds was effective but not complete. This is where growers make their second mistake. They believe that, since the weed looks dead, it won't be back. I believe you need to "kick" perennial weeds at least three times before they are under control. These three "kicks" must be when the weeds are vulnerable.
A common mistake is to say that "I will just increase the rate of herbicide to kill it." This does not work because of the dormant parts on the roots system. Even increasing herbicide rate will not get the herbicide into dormant weed tissue.
When you believe you have killed the weed, check the field thoroughly and you may still find low levels in parts of the field. This low level of weeds may not be enough to affect yield, but they will grow back if not controlled.
The above applies to most common perennial weeds in Ontario. Dandelions are a bit different, since so many of the plants start from air borne seeds. Vetch is different again. And the perennial weed nutsedge is different again. That is because it is sedge and has nutlets which can stay dormant for years. BF
Pat Lynch CCA (ON) is an agronomist for Cargill in Ontario.