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Weed Seeds Galore

Posted: July 27, 2016

Take a walk around your landscape and you'll see them everywhere. The weeds are up. I mentally divide my weeds into two major groups: ones that spread by seeds and ones that spread by roots and seeds.
Photo Courtesy Penn State Live

Photo Courtesy Penn State Live

All the annual and biennial weeds are in group one, the difference being that the biennials usually put down a short tap root and grow a rosette of basal leaves in year one and then flower in year two. The annuals go all out in one growing season, so they usually have fibrous roots.

Annuals and biennials have a strategy which is to dump as many seeds as possible in their short life. The seeds build up in the soil in what we call a seed bank, and then they wait until conditions are right to germinate. We help them out when we till the soil because tilling brings buried weed seeds up to the soil surface where they get the light they need to sprout.

I observed this first hand when I finally got all the brambles off my road embankment. The first thing that came up was false Solomon's seal (Maianthemum racemosum). I never saw that plant before. It's a native to North America, and it's been waiting all this time for enough light to grow.

Garlic mustard (Alliaria petiolata) (shown), an invasive biennial that is overrunning forest habitat, sends out up to 3,000 seeds from one plant and those seeds persist in the ground up to ten years. That's quite a seed bank. When I saw it blooming on my embankment this spring, I panicked. Since I don't see it anywhere nearby, maybe the seeds have been there for a long time and conditions this year were ripe for germination.

All of this information gives us a clue as to the weaknesses of plants that depend on seeds to spread. Our first line of defense is to stop them from making seeds. This doesn't require an herbicide. It requires a mower or any mechanical device that can take off the flower before it goes to seed.

In my opinion, it's best to let the plant flower because all its energy goes into that effort. If you cut the top off before the plant flowers, it may regenerate. Once it starts flowering, and this works for annual and biennial grasses as well as broadleaf plants, the plant is spent.

Now to that seed bank. The further down in the ground those seeds are buried, the lower the possibility of them ever germinating. In the garden, this means not turning the soil and also using mulch to deny light to any seeds near the top.

As I've improved my garden soil with compost and wood chip mulch, the soil has a looser structure. I can pull a weed with a short taproot (like dandelion) without breaking the root. Now I seldom get a dandelion in my beds. Instead, they have migrated to the compacted soil in my pathways.

Seeds fall on my garden each season, blown in by the wind or carried by birds. However, I keep mulching and burying the seeds, so most of them never sprout.

So what is the plan here? Don't let your weeds go to seed. Use mulch in your garden beds and avoid turning the soil. Pull weeds in the spring, but don't disturb your soil by pulling them once they get big. Instead, destroy their top growth with a weed torch, or mechanical means. I use a stirrup hoe that slices the top from the root at the crown. Finally, take heart. Think how good you'll feel when the weeds give up.

Contact Information:

529 Lentz Trail, Jim Thorpe, PA 18229 • 570-325-2788 • email

Eileen East, Penn State Master Gardener of Carbon County