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Weeds get spring fever too

Marissa Schuh, IPM Extension Educator

Models suggest that significant cold has passed, things are starting to green up and soil is warming. While this warm winter is causing growers stress about what diseases and insects will do this spring, this early spring can provide rare windows for weed control.  As with everything, what happens will vary at each farm and with each weed, but it might not all be bad news.

Early season weed flush. Photo: Tom Peters

For example, much of Minnesota had minimal snow cover for most of the season. For annual weeds that shed seeds, if those seeds were left on the soil surface, those seeds were easy picking for birds and rodents for most of the winter.

For those seeds in our soil’s weed seedbank, each species will be looking for different environmental cues to emerge. We are likely to see many weeds emerge earlier than usual. For example, common lambsquarters germinate when temperatures are as low as 43F, but most will germinate when daytime temperatures are between 64 and 77F. Other weeds will have higher rates of seed germination when daytime and nighttime temperatures are fluctuating (for example curly dock and common ragweed).

If things stay dry, we may have windows for stale seedbank technique that aren’t often available. While we know frost-free periods are shifting, we still are far away from frost free periods, especially for our warm season crops. Depending when you plan on planting and when your transplants will be ready, consider looking for windows well before planting to prepare your field and trick weeds into emerging at times when you can easily control them.

Perennial weeds will also likely emerge early, but again, things will vary by weed species.

Let’s look at Canada thistle as an example…

  • Canada thistle seeds germinate best when temperatures are warmer than what we have had (77-86F). 

  • Canada thistle broken up by fall tillage that spent the winter as a rhizome fragment are killed when exposed to 14-21F temperatures, so depending on if these fragments were on top of the soil or buried, winter could have been lethal or easy to get through.

  • Well-established plants pulling from deep root-systems probably rode the winter out fine, but will still need longer day lengths to flower. This may mean that we have more opportunities early in the season to exhaust storage roots through mowing or tillage while crops aren't planted yet.

Be observant and scout regularly as we go forward this season, as things are just going to be a little different.

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