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Aim for zero weed seed rain

Marestail with white fuzz on flowers, ready to spread seeds.
Marestail produces thousands of seeds. The white fuzz of the flowers means this plant is ready to distribute its seed.

While much of Minnesota desperately needs rain, there is one kind of rain gardeners should avoid: weed seed rain. Seed rain refers to plants sending out the seeds they’ve produced, be it by wind, inside the digestive tract of an animal, or by sticking to people or animals.

Seed rain, or seeds being dropped to the ground or dispersed in other ways, is something gardeners should worry about. Every time a weed goes to seed, dozens to thousands of seeds enter our garden soil, creating hours of future weeding.

As we enter the part of the growing season where many weeds are flowering and getting ready to set seed, it is a key time for us to pull and thwack weeds. Action now can keep them from dropping seeds into the garden.

Garden plants are in peak productivity, so are many weeds

Sprouted ragweed.
Common ragweed is responsible for allergies and frustration.

Many of the most persistent garden weeds are summer annuals, meaning they grow and produce seeds in the summer months, i.e. right now.

While you may see a certain weed species in the yard or garden every year, last year's annual plants die in the winter and their offspring germinate from seeds and come up the next year. Annuals complete their life cycles—sprouting from a seed all the way to seed production—in one year.

I’m busy, are a couple of weeds a big deal?

Green, berry-like fruit on and eastern black nightshade plant.
The immature fruit of eastern black nightshade will turn black when mature; each contains around 100 seeds. The fruit pictured here represent approximately 900 seeds or 900 future weeds.

This hot, dry growing season has been labor intensive, with gardeners of all scales spending a lot of time watering and doing tasks in the heat.

Now's the time to enjoy blooming flowers and juicy tomatoes, and it can be tempting to just focus on the fruits of your labor. But when it comes to weed management, the actions you take to manage weeds this year are going to greatly reduce the amount of labor needed to manage weeds in the future.

Many common Minnesota weeds can produce thousands of seeds. These seeds may germinate next year or, if they get buried deep in the soil, they will wait for the right conditions. Many weed seeds can lay in wait for years to germinate.

Every 20 years, a long-running experiment in Michigan digs up bottles of seeds buried in 1879. Seeds of moth mullein, dug up and planted earlier this year, were able to germinate after 140 years!

The table below includes common summer annual weeds, how many seeds the plant produces on average, and how long it takes for half of the seeds in the soil to die out. Note that these are just averages, and what happens in your garden will depend on weather, insects, soil bacteria, gardening habits, and more.

Weed Average seed production per plant Time to 50% depletion of seeds in soil 
Common lambsquarters 72,500 12 years
Common ragweed 3,500 1.5 years
Eastern black nightshade 10,000 5 to 8 years
Giant ragweed 10,300 <2 years
Marestail/Horseweed 200,000 Unknown
Velvetleaf 7,800 8 years

I have plants popping up everywhere, how do I know if I’m even looking at a weed?

Figuring out what should be pulled from the garden can be a challenge, especially if you are new to the space you are growing in. Many weeds are now large enough so that identification is much easier than when everything was just starting to sprout.

Proper weed identification is important, as different weeds are spread by different means, meaning some management actions work better than others.

The University of Minnesota’s “Is this plant a weed?” tool can help you narrow in on what you are seeing.

It looks like some weeds have already set seed, what do I do now?

Avoid letting weed seeds get buried in the soil. The longer they sit on the surface of the ground, the more likely they are to get eaten by birds, rodents, or insects.

Fans of gardening gadgets can use weed burners to kill seeds on the soil’s surface.

Make a note in your garden journal or on your calendar to do a round of weeding right after the fourth of July to prevent this occurrence in the future. 

Can’t I just spray something?

Velvetleaf plant with large seed pods and small yellow flower buds.
This velvetleaf plant is getting ready to flower, meaning it’s a good time to remove the plant.

This is not a good time of year to apply weed-killing products for a couple of reasons. The primary one being that many spray products don’t work on large, emerged weeds.

  • For example, trifluralin, the active ingredient in products like Preen, only works on weeds as they are germinating. This means a product with an active ingredient of trifluralin won’t do anything for the weeds you can already see in your garden.
  • Even products containing glyphosate are most effective on weeds shorter than a pop can. The size of many weeds makes herbicides a poor choice for control.

The other reason that spraying weeds right now isn’t a good idea is the risk products pose to the plants you actually want in your garden.

  • Many herbicides aren’t selective, meaning they are capable of killing weeds and desirable plants.
  • Summer’s heat can make application tricky, as many herbicides are prone to drift when temperatures are above 80 degrees.
  • Don’t assume that using an organic weed killer means there is no risk. These products often rely on various acids to kill plants, whether they are a weed or not.

The label of the product you’re thinking about using is the best source of information to find out if the product at hand will get the job done safely. Remember, the label isn’t a suggestion, it is the law.

What do I do with the weeds I’ve pulled or whacked?

It is best to get weeds out of your yard with a yard waste pickup. Composting can be an option if the weeds have not yet set seed. If you think the weeds have set seed, use caution when adding them to your compost pile or tumbler.

In order for most weed seeds to be killed, compost needs to reach an internal temperature of 130° F, though some weed species can only be killed when compost is held at a temperature of 145° for 30 days. Reaching these internal temperatures throughout the pile will involve careful management and regular turnings, so attempting to compost weed seeds might not be realistic for all home-composters.

For more tips on compost, see the University of Minnesota’s “Composting in home gardens”.

Author: Marissa Schuh, Extension educator, integrated pest management

Reviewed by Jared Goplen and Roger Becker

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