Driving through the southern Yukon, you might have noticed a tall branching plant that has taken over the roadsides in many places. Sweet clover is particularly hard to miss right now as it is in flower.

In the last few years sweet clover has taken root in many disturbed areas, and is thriving along many of our highways and byways. No one is sure exactly when it arrived in the territory, but the last two wet summers have definitely agreed with it, prompting dense growths in some places.

"It's probably been spread by vehicles coming up the highway; people bringing hay in from the south may have spread it," says Stu Withers, a Yukon botanist. He is investigating the spread of sweet clover around the territory, checking to see how far it has spread and whether or not it has become a problem.

Usually introduced plants such as sweet clover are not a problem in the Yukon as our long cold winters keep many exotic species from spreading. But that is no longer the case with both white and yellow sweet clover, with the white variety being the most abundant of the two.

Some southern jurisdictions have attempted to control sweet clover beside roads because -- with its dense high growth -- it can impair visibility for drivers. It could also attract wildlife to the roadsides, causing another type of traffic hazard.

Near Beaver Creek legumes such as clover were specifically kept out of the mix used for revegetating the roadsides just to ensure that caribou were not attracted to feed on it, but sweet clover has invaded this area as well.

The other concerns are that it could invade agricultural lands and wild lands, competing with other species. Withers is still gathering information, but says that so far sweet clover seems to be keeping to the roadsides and areas around communities.

"It is not competing well with natural vegetation, and it is not yet a major problem on agricultural lands. But it still could become a problem on naturally disturbed areas such as big alluvial fans," says Withers.

Sweet clover is native to Europe and Asia, but arrived in North America more than four centuries ago and now is found in most parts of the continent.

It is not considered to be a weed by everyone. In parts of North American it has been cultivated as a forage crop and as a soil builder since the early 1900s. It is also grown as a food source for honey bees.

Withers says the sweet clover appears to be most abundant along the Klondike and Alaska Highways. So far it has not spread very far up the Dempster Highway, and there is not much of it on the Atlin Road as the dense, wild vegetation growing close to the road leaves little room for invaders.

There are three different methods for controlling sweet clover; it can be burned, killed with herbicides, or mechanically controlled by pulling or mowing. Each method has its plusses and minuses, but all must be applied for at least two years in a row.

"The key to controlling sweet clover is that it is a biennial. The first year all of its energy goes into making roots and the second year it flowers, sets seeds and dies. If you burn it in the fall, for example, that just germinates the seeds in the ground and you get a thicker growth the next year."

In Minnesota and Wisconsin burning is usually used to control sweet clover, while it Illinois it is considered to be a serious management concern and is controlled with herbicides. Sweet clover is of particular concern around prairie preserves where its height makes it stand out when flowering.

In the Yukon, Withers says that decisions on whether to do anything about the spread of sweet clover remain to be made, though the highways department is concerned that it could be a visibility hazard because it grows so high. The Northern Research Institute funded this study, which will recommend establishment of an experimental program for testing different control techniques.

For more information, Stu Withers can be contacted at withers@internorth.com or at (867) 393-2884.