Lessons From a Project to Improve Prairie Quality – Part 2: Overseeding and Seedling Plugs

Last week, I posted a summary of some findings from a long project to enhance prairie habitat.  I focused that post on the lessons we learned from the fire/grazing management portion of the project, including impacts on regal fritillary butterflies.  This week, I’m looking at the other half of that project – overseeding and adding seedling plugs to our degraded prairies in order to increase plant diversity.  As with last week, you can find all the gritty details, including graphs, tables, and more, by looking at our full final report.

Maximilian sunflower is one of the species we've found easiest to establish in degraded prairies.

Maximilian sunflower is one of the species we’ve found easiest to establish in degraded prairies.  (These particular sunflowers are for illustration only – not from an overseeded site.)

During the five years of the project, we overseeded approximately 500 acres of prairie – focusing mostly on degraded remnant (unplowed) prairies that were missing many characteristic prairie wildflower species.  We harvested our own seed from nearby sites, and broadcast it on degraded prairies right after burning them.  The prairies were managed with patch-burn grazing, so cattle grazed those burned areas intensively for the remainder of the first growing season and then focused their grazing elsewhere in subsequent years.  To measure success of the seedings, I used replicated plots to count the number of new plants that established from seed.  Most of the seedings included multiple seeding rates, so I was able to look at the effect of seeding rate on establishment.

In addition to overseeding, we raised and transplanted more than 800 prairie and wetland seedlings into seven different sites, and added several hundred more seedlings to our nursery beds for seed production.  Most transplanting was done in the late spring, and plants were watered on the day of transplanting but afterward.  We marked (GPS and flags)and attempted to re-locate seedling plugs to evaluate survival, but that didn’t work out very well, and we didn’t find a lot of the plants we’d plugged in.  Some of those plants surely died (which prevented us from finding them), but for others, flags disappeared and GPS points weren’t accurate enough to lead us to the small plants we thought were probably there.  We did find some, but our estimates of success are pretty fuzzy.

We learned two major lessons from this portion of the project:

1.  Overseeding after a burn in a patch-burn grazed prairie can re-establish at least some missing plant species, but the use of a high seeding rate is important.

2.  Overseeding seems to be more cost effective than seedlings, assuming abundant seed can be obtained relatively cheaply.

 In tallgrass prairies further to the east of us, people have had pretty good, if inconsistent, luck with overseeding prairies without necessarily having to suppress the vigor of surrounding vegetation.  We’ve tried that here, and have seen very low success, maybe because our drier climate (25 inches of precipitation per year) increases competition for moisture?  Regardless, our best results have come from seeding after a burn – for good seed/soil contact – followed by grazing of the dominant grasses that appear to be the primary competition for new seedlings.  Patch-burn grazing works well, but we’ve also had good luck in the past by just grazing intensively for a month or so after seeding, and then pulling the cattle out.

Trails from our ATV and broadcast seeder in recently burned prairie.  Broadcasting after a burn helps get the seed/soil contact we need.  Experiments with light harrowing as a way to get evern more soil contact haven't provided any measurable benefits.

Trails from our ATV and broadcast seeder in recently burned prairie. Broadcasting after a burn helps get the seed/soil contact we need. Experiments with light harrowing as a way to get evern more soil contact haven’t shown any obvious results.  Note the absolute straight lines I made as I planted this site… 

Seeding rate was very important.  We started by seeding at about the same rate as we use when we converting cropland to high-diversity prairie – about 1-2 lbs of bulk forb seed per acre.  As the project went on, we went as high as 8 lbs, and continued to see better results.  At least in our prairies, seeding smaller areas with more seed seems to be more effective than spreading limited seed over large areas.

Because others have incorporated light tillage or harrowing to suppress competition and increase seed/soil contact, we tried some of that as well, but our results were mixed.  Some tilled plots showed very high establishment, but others showed less than non-tilled plots.  We did find that when we tilled a few inches deep, we didn’t seem to kill any plant species – remembering that these are degraded sites already.  I would definitely not recommend that others try tillage on a large scale, but in small plots within degraded grasslands, it’d probably be worth some more experimenting.  We had a beautiful set of replicated tilled plots that I hoped would clarify the situation in 2012, but the severe drought overwhelmed that attempt.

Even at our highest seeding rates of 8 bulk pounds of forb seed per acre, the density of established plants was relatively low (in our best sites, we established around 150 new plants per acre) but hopefully high enough to create self-sustaining populations that will grow over time.  The plant species that established most readily included:

  • Maximilian sunflower (Helianthus maximiliani)
  • Sawtooth sunflower (Helianthus grosseserratus)
  • Stiff sunflower (Helianthus pauciflorus)
  • Illinois bundleflower (Desmanthus illinoensis)
  • Entire-leaf rosinweed (Silphium integrifolium)
  • Wild bergamot (Monarda fistulosa)
  • Black-eyed Susan (Rudbeckia hirta)
  • Purple & white prairie clover (Dalea purpurea and D. candida)
  • Canada milkvetch (Astragalus canadensis) – in some sites
  • Illinois tickclover (Desmodium illinoense) – in some sites

In terms of seedlings, we have found that most prairie plants are easily grown in greenhouse situations (with some exceptions) but that some take more than a year to germinate, and then perhaps a full year or more to grow large enough to transplant.  When we planted the seedlings into prairies, we clumped them together in groups of 5-10 plants to help form populations that could cross pollinate, and to make it easier to find at least one of the plants we’d put in.

Compass plant seedlings and others in our greenhouse.

Compass plant seedlings and others in our greenhouse.

We had success with seeding plugs in some situations – particularly in terms of getting wetland sedge species established in restored wetlands – but transplant survival in degraded mesic prairies was mixed at best.  Most of our transplanting was done in the late spring, as we hoped to synchronize our planting with the wettest time of the year, but we may experiment with more fall planting in the future.  We felt that many of our seedlings may have died because they weren’t in the appropriate soil conditions, which we had to guess at since there were no existing populations of most of the species we were transplanting.  Broadcasting seed is probably a better way to match up appropriate plant species with their specific microhabitat requirements.

In our situation, it appears that overseeding is a cheaper and more efficient way to increase plant diversity in degraded prairies.  Of course, one big reason it makes sense for us is that we have existing capacity for large-scale seed harvest.  If enhancement of degraded prairies is a high priority for a landowner or land management entity, it might make sense to build their own seed harvest capacity.  That doesn’t necessarily mean large investments in equipment or people, though a pull-behind seed stripper or combine can be a nice way to harvest large amounts of seed quickly.  Large amounts of wildflower seed can also be harvested by hand (our typical method) if you are efficient and organized.  More information on seed harvest techniques and equipment needs can be found here.

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About Chris Helzer

Chris Helzer is an ecologist and Eastern Nebraska Program Director for The Nature Conservancy. He supervises the management and restoration of approximately 4,000 acres of land in central and eastern Nebraska - primarily along the central Platte River. Chris is also the author of "The Ecology and Management of Prairies in the Central United States", published by the University of Iowa Press.
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13 Responses to Lessons From a Project to Improve Prairie Quality – Part 2: Overseeding and Seedling Plugs

  1. Steve says:

    More interesting stuff Chris, thanks for sharing.

  2. James C. Trager says:

    These are hopeful findings.
    Yeah, and … Those lines may not have been straight, but at least they were parallel (+ or -).

  3. Jeff says:

    We had a local Eagle Scout do some transplanting into a species poor native grass seeding and have had fairly good results. We purchased plugs (50 to 100 of each species) in early August 2010 which the Eagle Scout planted in his grandfathers garden and overwintered them there. We burned the site (five acres) the first week of April the following year, and the plugs, almost all of which were still dormant, were planted into the site the third week of April by the Eagle Scout’s crew. Species planted included prairie violet (no establishment) compass plant ( good – 75% or better survival) prairie phlox (Fair – 50% survival), silky aster (good), narrow leaf purple coneflower (poor less than 10% survival) prairie onion (fair), and prairie coreopsis (good). Phlox, aster, onion, coneflower and coreopsis have all flowered and the compass plant should this year or next. Plants were marked with wire and plastic marking flags. I have also had good success transplanting liatris corms into existing seedings, with prairie blazingstar being the most successful.

    • Chris Helzer says:

      Thanks Jeff. Where’s the site at?

      • Jeff says:

        Renville County in southwest Minnesota. The thing I forgot to mention in the earlier post is that we also had an abundance of rainfall for the first two months after planting that probably helped the plugs establish.

        • James McGee says:

          Jeff, I am writing to inform you that Stephen Packard has achieved survival of Prairie Violet at a restoration site. Stephen was able to establish the prairie violets in old crop fields being restored where competition was still minimal. In contrast, at another site prairie violet was planted among tall grasses and no survival has subsequently been observed. At Neil Smith National Wildlife refuge they have succeeded in establishing prairie violet in their restorations which contain tall grasses. I’m not really sure why some are successful and others are not. It really seems to be hit and miss.

          James

  4. Chris,
    Due to the high variability in seed sizes for native species (tiny Juncus or Ludwigia vs heavier Desmanthus or Silphium) and the fact that I usually deal with seeding 1 acre or less (sometimes less than 100 square feet!), I prepare mixes with a target number of seeds per square foot. Do you have a feel for the seeds/lb in your mixes? I’m trying to figure out where we fit in your scale of low vs. high seeding rates.
    Thanks for another great post!

    • Chris Helzer says:

      Hi Danelle,

      I wish I had a better answer for you. We decided not to spend the money on seed tests, but the disadvantage is that we can’t compare mixes very easily. What I can tell you is that most of the seeds in the mixture are fairly large. Black-eyed susan is pretty small, but most of the mix consists of legumes (Dalea, Astragalus, Desmanthus, Desmodium), sunflowers, and silphium. Sorry I can’t be more specific.

  5. James McGee says:

    Chris,

    -I suggest using colored aluminum tags to mark your plugs. This should help you relocate them.
    -Sowing seed is generally cheaper than growing plugs. We only grow plugs of those species for which seed is extremely limited. The plugs then get planted into seed gardens so we can obtain larger quantities for sowing.
    -Planting plugs has the advantage of being a favored activity of volunteers. This activity is a good way to introduce new people to volunteering, even if sowing seed is more economical.
    -I would not suggest fall planting. The potting mixes used to grow plugs absorb more water than the native soil. When the soil freezes, the extra water in the plug causes it to expand. This pushes the plug out of the ground exposing the crown of the plant to all weather extremes. This is typically fatal.
    -Finally, Over seeding is something that should be done over a number of years. This should limit any single growing season from having too significant of impact on the results.

    Sincerely,

    James

  6. Keith Long says:

    Hi Chris – I am grazing cattle in the Flint Hills in and around a wind farm. On hot days, the cattle tend to use the turbines for shade and line up behind the turbine and follow it around with the sun. We use a pretty intensive grazing program, and get quite a bit of recovery time before coming back to that same area (100+ days), but still, the only thing that grows around the turbines are annual broom weed and lanceleaf ragweed. What are your thoughts about seeding those areas, and what would species would you use? It is even worth the effort?

    Thanks
    Keith

    • Chris Helzer says:

      Keith,

      My best guess is that seeding won’t do you any good unless you can change the grazing pattern (can you get them to put clear glass panels in those turbine towers?) : )

      The broomweed and ragweed are responding to the grazing pressure, and they’re the species best suited to that kind of grazing pressure. The broomweed and ragweed aren’t outcompeting other species, they’re just filling in the open space created. I don’t know of anything you’d want to seed there that would outcompete those species in those conditions (at least nothing you’d want). I’m sure you’re talking to others more local than me, which would be smart, but that’s my best guess from here. The upside is that the broomweed and ragweed are pretty awesome habitat for some wildlife species (brood-rearing habitat for upland game birds, for example)

      • Steve says:

        This struck me as an interesting contemporary version of the classic range management problem – distribution of animals (i.e animal impact) in a management unit. The classic tools used to encourage even distribution throughout a management unit have been the placement of water and salt/minerals/supplements to draw animals towards underutilized areas and away from overutilized areas. Cross-fencing/rotation is also a tool used to address distribution issues (uneven distribution in the short-term, even distribution in the long-term) as is the application of fire over the whole unit (encourages even distribution during the growing season following the fire). A relatively novel tool is the uneven distribution of fire in a portion of a management unit in one season or year, but shifting that application of fire through the other portions of the management unit in subsequent seasons and years (i.e., patch burn grazing – distribution is uneven in the short-term but even in the long-term). What Keith is dealing with is the relative effect of thermal refugia (i.e. shade) on animal distribution vs the other things that influence distribution (forage quality and quantity, water, slope, etc.) and shade is a very powerful influence on cattle distribution.

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