Home Clippings Dandelions Muscle Into Annual Field Crops   by Gord Leathers

- Gord Leathers photo Dandelions wouldn't be so dandy if you can ht them with a systemic herbicide in the fall, before they set themselves up to overwinter, students heard at this year's Crop Diagnostic School.

Dandelion has become an increasing prob- lem for Manitoba farmers, especially in field crops according to University of Manitoba weed ecologist Rene Van Acker. Traditionally it was a problem in perennial forage crops such as alfalfa but is becoming more and more com- mon in field crops. "They're a significant problem that's risen in prominence since the last survey, which was in '97," Van Acker said at the provincial Crop Diagnostic School last month at Carman. "It's hard to control and it's especially hard to control in the spring." The good news is, the dandelion has an Achilles heel. It's vulnerable at the end of the season, so fall control can be very effective. Its seeds are also relatively short-lived, so two years of effective fall control can significantly reduce field populations. Iron-tough and difficult to kill, the dandelion produces vast numbers of windblown seeds that take full advantage of wherever they land , and hunker down for years once established. "Dandelion is a simple perennial that doesn't spread by root or rhizomes," Van Acker explained. "It spreads by seed so knowing what's happening with the seedlings can tell us something about how to prevent the spread of the species." The dandelion overwinters as a rosette - and overwintering plants-get spring populations going. They come up early before any in-crop spraying. According to studies by Kim Hacault, one of Van Acker's graduate students, the spring rootstalk plants showed 50 per cent emergence after 450 growing-degree days, which puts them in the air in the last week of May. On the other hand, the seedlings come on much later, somewhere around mid to late June. The question was, why should the seedlings emerge so late? The answer is in the dandelions' seed bank - that's a cache of seeds that stays on the ground and germinates when the conditions are right. "We looked for a seed bank and found that dandelions don't have much of a seedbank. We found that only four Percent of the seedlings were from a true seedbank," Van Acker said.

"Ninety-six per cent were from seeds that were shed that year, So seeds that were shed that year are very important. Stopping dandelion flowering in the spring is a worthwhile exercise." The first and probably most effective way to do so is to kill them off In the fall before they set them- selves up,to overwinter when the plant is vulnerable. "That's when the plant is starting to put its reserves back into the rootstalk or back into those organs that it's going to regenerate from," Van Acker said. "If you're using systemic herbi- cide, you get herbicide moving into those areas and it starts to kill off those buds." A fall kill presents a number of advantages, First, it only requires a three-quarters-litre-per-acre appli- cation of glyphosate, a relatively inexpensive herbi- cide. Second, the initial wave of plants is taken out, so the next wave of spring seeds simply isn't there. This is the second weakness of the plant. The seed bank is not long-lived and since only four per cent of the seedlings come from overwinter- ing seeds, taking out last year's source can put a crimp on this year's population. But the dandelion has a couple of ecological tricks up its sleeve. Because the rootstalks emerge , early they're mature during the in-crop herbicide application and continue on untouched. Since the seedlings emerge late, they're germinating after the spring herbicide application so they hunker down between the crop plants and live to set seed. "If we're trying to prevent that population from spreading we don't do that with our spring application pre-seeding and we don 't do it with our in-crop application," Van Acker said. "It has to be a fall application and we've found that fall applications were much more efficacious than spring applications." Dandelions are also vulnerable to spring tillage and they don't like having their roots dis- turbed.This can reduce their biomass production later in the ear by up to 50 per cent. "We've found a single tillage pass in the spring has a very significant effect," Van Acker said. "So if you are a conventional tiller you might want to go for high-disturbance direct seeding rather than low-disturbance direct seeding and that can be effective as well."