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Greenspace: Does that bag of seed include weeds?

January 14, 2019
A seed analyst must be able to visually identify all seed to species including common and noxious weed seeds. In some cases it is not possible to identify noxious weeds by visual analysis only. For example, a genetic test must be used to identify Palmer amaranth.

Buying bags of seed to prep for the spring?

Not all seed bags are created equal. And before you begin sowing willy-nilly, it’s well worth the time to check your bag’s label.

Labels should indicate the percentage of seed in a given bag that’s likely to sprout if planted, the “kinds” or species of seed present in the bag, and the amount of pure seed, or the seed that’s untainted by other types of matter like chaff or weed seed, says Michael Merriman, of the Minnesota Department of Agriculture, in a press release.

It’s the weed seed we’ll be talking about today.

On your label, you may see something like “Weed Seed: 0.20’ or a section that says “Noxious Weed Seed,” followed by an amount, which could be “none.”

The weed seed measurement shows how much of the bag includes seeds that are considered weeds in agricultural or natural settings.

In Minnesota, the percentage of weed seeds shouldn’t exceed 1 percent of the contents of a seed bag. And that doesn’t take into account the presence of restricted or prohibited noxious weed seeds.

Some weeds are considered noxious or are detrimental to human or animal health, the environment, crops, livestock, or other property. They come in two varieties: restricted and prohibited.

Restricted noxious weeds can be sold if their percentage is below the specified acceptable rate. But if prohibited noxious weeds are found in a bag of seed, the seed should not be sold.

Restricted noxious weed seeds are allowed in seed at a rate of 25 seeds per pound, Allen Sommerfeld of MDA Communications said.

Restricted noxious weed seeds include the following: Buckhorn plantain (Plantago lanceolata L.), Dodder (Cuscuta spp.), Frenchweed (Thlaspi arvense L.), Hoary alyssum (Berteroa incana D.C.), Horse nettle (Solanum carolinense L.), Wild mustard (Brassica arvensis L.), Quack grass (Agropyron repens L.), Wild radish (Raphanus raphanistrum), Giant foxtail (Seteria faberii), and Eastern black nightshade (Solanum ptycanthum Dun).

Prohibited noxious weed seeds include these: Bull thistle (Cirsium vulgare L.), Canada thistle (Cirsium arvense Scop.), Musk thistle (Carduus nutans L.), Perennial sow thistle (Sonchus arvensis L.), Plumeless thistle (Carduus acanthoides L.), Field bindweed (Convolvulus arvensis L.), Hemp (Cannabis sativa L.), Leafy spurge (Euphorbia esula L.), Perennial peppergrass (Lepidium draba L.), Russian knapweed (Centaurea repens L.), and, most recently, Palmer amaranth (Amaranthus palmeri S. Wats.).

Because weed seeds can’t be visually identified, harvested seed for sale is typically cleaned and conditioned by moving the seed through screens to separate out any unwanted material. Seed that is the same size or weight as the desired crop is difficult to sort out, which is why seed testing is also done to determine which weed seeds are present.

A seed that is not legal for sale in Minnesota may be legal in another state because each state maintains its own noxious weed seed lists, Sommerfeld said.

A note: Small seed packets for home gardeners are not required to have weed seed information labeled as long as the pure seed percentage is greater than 90 percent, the inert matter is less than 10 percent, or other crop seed, weeds, or noxious weeds are not present in the package.

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