Use seed adapted to local climate and growing conditions

Yellow tag

Iowa yellow tag certified source-identified seed is highly recommended for roadside plantings. Because the Iowa Crop Improvement Association certifies that this Iowa seed originates from Iowa prairies, the region and climate to which it is adapted are known. This seed is often collected from multiple sites within a region of the state, giving it a broad genetic base and potentially making it adapted to a wider range of growing conditions. Buying yellow tag seed also reduces the risk of introducing new weeds to Iowa. Most of the seed provided through the FHWA grant is yellow tag seed.

Local ecotype

Local ecotype seed of Iowa or nearby origin that is not yellow-tag certified is also appropriate for roadside plantings and can be obtained from seed vendors by requesting “local ecotype” seed. Some counties establish their own local ecotype prairie grass and wildflower production plots with seed collected from prairie remnants within their county or region. It is important for this seed to be well cleaned and tested so it is known how much live seed is actually being planted.

Cultivars

Cultivars or cultivated varieties are generally not recommended. Cultivars are often derived from sources too far south and west of Iowa and so are adapted to a different climate and growing season. Most were developed for forage production and can be too aggressive in diverse plantings. Cultivars are available in only a limited number of species.

Sources of yellow tag species and related information can be found in the Iowa Crop Improvement Association’s Native Seed Directory. Additional native seed sources and information can be found in the Tallgrass Prairie Center’s Iowa Prairie Seed and Service Providers page.

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:

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

Keep seed cool and dry

The viability of native seed deteriorates rapidly at high temperatures and high humidity.

General rule of thumb for seed storage: Temperature plus humidity should not exceed 100.

• Most seed will last at least a year at 50° Fahrenheit and 50% relative humidity.
• For each 10° increase in temperature, seed longevity is halved.
• For each 1% increase in moisture content of the seed (not RH), longevity is also halved.

Example: Seed stored at 70° and 6% moisture content has only one-quarter the lifespan of seed stored at 50° and 6% moisture. Likewise, seed stored at 50° and 8% moisture content has only one-quarter the life span of seed stored at 50° and 6% moisture.

“Ideal” seed storage requirements vary with individual species, but most can be stored in a temperature- and humidity-controlled environment for at least a year without losing significant viability. Some IRVM programs have a dedicated seed storage facility. These insulated rooms and small buildings are rodent proof and include air-conditioning units and sometimes industrial dehumidifiers.

In the absence of a seed storage facility, seed should be stored in the coolest place possible. Air circulation can improve conditions in spaces without temperature/humidity controls. Short periods of heat (over 100° F) can be tolerated by most seeds, but long-term exposure can destroy the embryo.

Commercially produced seed has been properly dried before being bagged. Ideally, seed storage bags should be made of breathable materials such as cloth or woven nylon. Well-dried seed (8–14% moisture content, depending on species) — if  kept cool and dry — can be bagged and stored in garbage cans, plastic bags or other sealed containers without suffering damage from fungus or freezing. Watch for moisture build up.

Additional information on seed quality, processing and storage is found in The Tallgrass Prairie Center’s Native Seed Production Manual.

Roadside Manager Insights

We’ve noticed Liatris grows very poorly unless it’s dormant seeded shortly after harvest. Perhaps it loses viability or vigor when stored over winter, or maybe it needs to be stored at a higher humidity than most seed. 
-Jim Uthe, James Devig, Dallas County, 2024

With a little increase in rate per acre, I think generally year-old seed can be used with no problem. We use older seed (2–3 years) at whatever rate it takes to use it up over the course of the season. I add a reduced rate of new seed to cover any loss of germination. 
-Linn Reece, Hardin County, 2010

Take advantage of prairie diversity

Native roadside seed mixes need to include species adapted to a wide range of growing conditions, from wet to mesic to dry. To outcompete weeds, the mix should also include species that occupy different ecological niches within the planting, grass species and broadleaf species, warm-season and cool-season species, tall plants to shade out thistle seedlings, and small plants to fill in underneath.

When working in narrow ditches, such as those found within a 66-ft. right-of-way, it is most efficient to design one mix that includes species for a wide range of site conditions — from the gravelly, well drained soils at the top of the slope, to the heavy, saturated soils at ditch bottom. Apply the same mix over the entire area and let it sort itself out. Wider rights-of-way may have wet or dry areas large enough to justify designing and planting a seed mix specific to those spots.

To achieve a well-rounded mix with all the benefits native vegetation has to offer, include species from each of the following functional groups. The Tallgrass Prairie Center’s Designing Seed Mixes technical guide and seed calculator can also be helpful for designing seed mixes.

A well-rounded native seed mix will include species from each of these groups:

Quick-establishing

Some native species develop faster than others. These are important for early erosion control and provide positive PR while slower species establish. 

Warm-season grasses

Roadside plantings rely heavily on these prominent members of the native plant community. These grasses continue to grow through the hot summer months. They provide long-term erosion control and good fall color.

Cool-season species

Plantings are strengthened by a species component that greens up early in the spring. These plants provide late-winter/early-spring erosion control and occupy the niche sought by non-native, cool-season competitors like smooth brome.

Legumes

The prairie flora includes many legumes that thrive in roadside plantings. They fix nitrogen and improve habitat.

Showy and easy

These species establish readily, are relatively inexpensive and create masses of color noticeable at 65 mph. They are crowd pleasers. 

Showy

The following species cost a little more or are harder to establish but still add important color and habitat.

Early bloomers

It’s relatively easy to extend a planting’s blooming season into the fall. Spring color is harder to come by. The following plants provide the earliest color visible from the road.

Wet species

Upland species dominate roadside seeding mixes. Species adapted to wet areas are needed for moist ditch bottoms.

A note about tall grasses and non-native perennials

For visibility and safety, tall grasses — such as big bluestem and Indiangrass — should not be planted at intersections or driveways. Omitting these grasses throughout the remainder of a planting, however, will limit its adaptability. Big bluestem and Indiangrass are versatile, adapted to conditions from medium-dry to medium-wet. Short native grasses, on the other hand, are generally adapted only to dry sites.

Non-native perennials — such as tall fescue, perennial rye, crown vetch, and birdsfoot trefoil — are very competitive and will persist to the point of adversely affecting survival of native seedlings. These species should not be used in permanent or temporary mixes for sites planted to natives.

Seed mixes for shoulders

Native species are not used on the shoulder. Examples of seed mixes that withstand repeated mowing and are well-suited to shoulder conditions follow.

• 50% Kentucky, 31% fescue, and 50% perennial rye

• 45% Kentucky, 31% fescue, 45% perennial rye, 6% medium red clover, and 4% alsike clover

• 35% Fawn fescue, 35% perennial rye, 20% timothy, and 10% alfalfa, or hairy vetch

Roadside Manager Insights

Keep in mind in most situations we’re not recreating a diverse prairie. We’re stabilizing the roadside with native plants.

-Joe Kooiker, Story County, 2024

Aspect/shading sometimes need to be taken into account, especially on small plantings or problem areas. In shaded areas, we use savanna species in the mix. If there’s a lot of shade, we may plant a nonnative, cool-season mix. 

-Jim Uthe, James Devig, Dallas County, 2024

We use local ecotype hand-harvested forbs and combined seed from our native seed nurseries to add diversity to our roadside seed mixes. 

-Joe Kooiker, Story County, 2024

Try to tailor your mix so the ROW has color all year long.

-Joe Kooiker, Story County, 2024

Depending upon soil type, we may use up to a 1.5x the rate of native seed to help speed establishment and stabilization. 

-Jim Uthe, James Devig, Dallas County, 2024

Seeding rates

Put down enough good seed to get timely vegetative cover and ensure planting success

Steeper slopes require heavier seeding rates. To get adequate erosion control, it’s more affordable to increase the amount of grass in the mix than to increase forbs. The result is that roadside plantings tend to have a higher grass to forb ratio than other prairie restorations. No matter how much native grass seed is put down, at least a 25% forb component is recommended to achieve adequate diversity and long-term stability. A 50% forb component is considered adequate for a fairly diverse planting. Some counties—especially those with their own forb seed plots—may exceed 50% for intersections and other highly visible plantings.

Grass to forb ratio

To get adequate erosion control, it's most affordable to increase the amount of grass in the mix. No matter how much native grass seed is put down, at least 25% forb component is recommended to achieve adequate diversity and long-term stability. 50% is better.

Seeding rates are calculated in one of two ways:

• Seeds per square foot 
• Lbs./acre

Though frequently used, lbs./acre is not a precise way of measuring the number of seeds planted since seed weights vary greatly between species. For example, one ounce of compass plant contains 660 seeds, while one ounce of black-eyed Susan contains 92,000 seeds. When accurate calculations are desired, use seeds/ft.

Seeding rates for roadside mixes are determined by slope and – to a lesser degree – seeding method. Budgets can also be a factor. The following chart provides general guidelines – a good idea of how many seeds should be put on the ground.

Minimum recommended seeding rates

These rates apply to drill seeding, broadcast seeding and hydroseeding (two-pass method). When hydroseeding with seed mixed in the slurry (one-pass method), increase rates by 15–30% to compensate for seed hung up in the mulch. (Note: Under real working conditions, even the best attempts at measuring seed quantities in the field will not be perfectly precise.)

The Iowa Prairie Seed Calculator will help you if you would like to create custom mixes.