Seed Labels

Learn to read them

Commercial seed labels contain a variety of information related to the quality of the seed. Some or all of the following items will be on the label:

  • Pure seed (purity) — Percent of material in the bag that is actually the desired seed
  • Inert matter — Percent plant debris or other materials that are not seed
  • Other crop seeds — Percent non-weed seeds
  • Weed seeds — Percent seeds considered weed species
  • Name and number of noxious weed seeds per pound
  • Germination — Percent of seed that will germinate readily in a germination chamber
  • Hard seed — Percent of seed that does not germinate readily because of a hard seed coat
  • Dormant seed — Percent of seed that does not germinate readily because it requires a pretreatment or weathering in the soil. (Some suppliers may combine hard and dormant seed on the label.)
  • Pounds pure live seed (PLS) — (# PLS) = (# bulk) x (% purity) x (% germination + % dormant)

A “TZ” (tetrazolium) % may also be on the label. Some native species’ seeds will not break dormancy for germination tests. These seeds can be biochemically tested using tetrazolium chloride (TZ). Living tissue is stained red, allowing analysts to determine the viability of non-germinated seed.

Seed stored for more than a year or grown/harvested “in-house” should be tested. The Iowa State University Seed Testing Laboratory and many private seed testing labs perform TZ tests as well as purity and germ tests, and will identify weed seeds in the sample. TZ test kits are also available.

The amount of weed seeds in a lot can vary widely; some of these amounts can seem high but are not necessarily cause for concern.

Tallgrass Prairie Center Insights

Three hundred and eighty noxious weed seeds per pound of native seed does seem high, but I sometimes get lots that have over 1,000 noxious weed seeds per pound. In my mind it's negligible since the actual weed seeding rate ends up being very low, and they are not necessarily all live seeds. The noxious weeds that show up in these tests are almost invariably annual agricultural weeds, so they will probably be gone within 2–3 yrs. The weed seed bank (at least in post-agriculture settings) is many orders of magnitude higher than these small amounts. Weeds listed as noxious on the tag are not necessarily Iowa noxious weeds either, so depending on who you buy from they might not even need to report it that way. In other words you may be seeding these weeds without knowing their identity anyway (they would be listed as a whole in the weed seed percentage). In this example, wild buckwheat is noxious in Minnesota but not in Iowa. I'm guessing they included it to cover bases for interstate commerce. While I wouldn't reject a lot like this, it would be worth keeping an eye on the planting. 
-Justin Meissen, Research and Restoration Program Manager, UNI Tallgrass Prairie Center, 2024