DOT Project Number:  90-00-LRTF-505

Fiscal Year:  2005

Award:  $16,730.00

Principal Investigator:  Dr. Laura Jackson, Department of Biology, University of Northern Iowa, Laura.L.Jackson@uni.edu

Research Report:

ADDING WILDFLOWER DIVERSITY TO SPECIES-POOR GRASSLANDS: EFFECTS OF MOWING ON COMMON AND CONSERVATIVE SPECIES

Executive Summary

The Department of Transportation needs reliable methods to establish diverse roadside prairies with minimal erosion, weeds, and cost.  We investigated inexpensive ways to turn species-poor grass plantings, which are common along Iowa roadsides, into diverse prairie mixtures. Since its inception in 1999, this study has found that:

• It is possible to add wildflower diversity to established, species-poor grassland repeatedly, a few species at a time;

• Mowing every 1-3 weeks is required for the first addition of forbs to a grassland, but perhaps is not as important for subsequent seedings;

• Mowing the year of establishment tends to produce larger forbs that can better compete with grasses and flower more quickly.

Since 2005, attention has turned to establishing showy, expensive and conservative species to species-poor grasslands, with focus on Prairie Phlox and Midland Shooting Star.  Research in 2004 - 2006 has revealed that

• A limited number studies exist on germination of Prairie phlox or Shooting star;

• Prairie phlox germinates best in early spring (March), and is greatly inhibited by cold-moist stratification.  Second-year germination is much greater than 1st year germination.

• Shooting star may not germinate at all the first year, with or without cold-moist stratification.

• New seedings into burned sod may experience heavy seed predation by mice and birds; and

• Capsaicin effectively deters seed predation of Shooting Star seeds but not Phlox.

Ongoing studies in 2006-2007 are investigating whether germination of showy species can be improved by overwhelming seed predators (mice, birds) with extra food at the time of seeding, or by coating seeds with capsaicin, a known feeding deterrent of mammals and invertebrates.

We have described our forb enhancement techniques at the North American Prairie Conference (Madison, Wisconsin 2004; Kearney, Nebraska 2006), the Iowa Prairie Conference (Cedar Rapids, 2005), and other professional gatherings.  Approximately 200 people have visited our demonstration plots on the UNI campus, including roadside managers and NRCS employees.  We field numerous phone calls and emails about forb enhancement.  These contacts include the Linn County Roadside Manager, Indian Creek Nature Center, Wild Designs Landscaping, Iowa Prairie Network, Practical Farmers of Iowa, and the Midwest Organic and Sustainable Education Service.  We work closely with the Tallgrass Prairie Center and the Roadside Office at UNI to share information on planting techniques with Center Staff and roadside managers.

BACKGROUND

Since 1998, experiments at the University of Northern Iowa Prairie Preserve have been devoted to forb enhancement of species-poor grasslands.   Species-poor grasslands are defined as prairie plantings dominated by warm season and cool season grasses, with wildflowers occupying less than 2% of the cover (Williams 2002).  These grasslands, which are common throughout the state, are inferior at excluding noxious weeds, provide poor wildlife habitat, and do not provide the aesthetic qualities expected from a successful roadside planting.

Initial experiments by Dave Williams (1998-2000) showed that it was possible to add forb diversity to a grass-dominated prairie reconstruction, simply by overseeding and mowing every week for a single growing season.  Currently healthy stands of Black-eyed susan, Stiff goldenrod, Grey-headed coneflower, Pale purple coneflower, Showy tick trefoil, and Bee-balm bloom in Williams' formerly mowed plots, where only grasses once grew (Figure 1).  Following his success, a new experiment was begun in Spring 2003 using crucial seed money from LRTF.  With major support from LRTF in 2004, the project was expanded.

We wanted to see if we could further enhance Williams' plots by adding ten new, more conservative forb species (Table 1).  Plots were assigned to one of three mowing treatments either control, mowed infrequently or mowed frequently for summer 2003 and 2004.  Mowing ceased in 2005, allowing us to assess forb abundance, size, and maturity (flowering status) as a function of experimental mowing treatments.

A study was begun in spring 2005 planting prairie phlox and shooting star at a seed density of 15,000 seeds/m² (1,400 seeds/ft²) to study effects of four different seed treatments on seed germination. We wished to improve success at planting these two expensive, hard-to-establish species. We found very poor germination of both species during the first growing season. Upon further investigation we discovered many of the seeds were absent from the soil during subsequent seed recovery attempts. This led to a few interesting questions about the fate of those seeds:

• Were those seeds consumed by seed predators such as rodents and ants?

• Were the seeds dormant, and possibly entered the seed bank?

• Have the seeds died in soil and will never germinate?

After an extensive literature search, we discovered seed predators appeared to be a possible reason for the seed disappearance and resulting poor germination rates. Previous work has shown rodents are capable of removing 50-95% of seeds (Heithaus, 1981; Sullivan, 1982; Nolte, 2000). This amount of granivory can drastically reduce the amount of new plant recruitment in a revegetation project. With money from LRTF we were able to expand on this problem and study possible means to reduce the amount of damage seed predators can have on a seeding project.

PART I (Incremental Forb Enhancement)

• Can we continue to add forb species incrementally, when there are already a few species of forbs in the grasses, or will mowing kill the existing forbs? 

• Did mowing increase the size of seedlings of the ten new species added during the second incremental seeding?

• Is it possible to mow less frequently and get the same beneficial results?

• Do some forb species respond better to this technique (mowing) than others?

PART II (Addition of Conservative, Showy and Expensive Species)

• Does planting time affect the success of more conservative species?

• Do pre-planting seed treatments affect germination in conservative species?

• Are non-germinating seeds remaining in the seedbed or lost through predation?

METHODS

PART I (Incremental Forb Enhancement)

Experimental Design

The setup for this experiment was a randomized block design consisting of two 60m x 60m blocks, each containing twelve, 15m x 20m plots.  In the fall of 2002 the experimental area was burned to prepare the area for a broadcast seeding.  In March of 2003, 10 species of native prairie forbs (Table 1) were hand seeded into 18 of the 24 plots at a rate of 25 seeds/m² per species.  Three different mowing regimes were randomly assigned to each of the 18 seeded plots.  The treatments consisted of an unmowed control, mowed infrequently (mowed every three weeks) and mowed frequently (mow every week).  The mowing of the plots began in summer of 2003 and continued through the summer of 2004.  The plots were mowed at a height of 15cm which gradually increased over the two growing season to a height of 20cm by the second growing season to avoid clipping any growth on newly establishing seedlings.  Mowing ceased after fall of 2004.

Plant Size

During the third growing season (2005) the seedlings that had emerged in 2003 and 2004 were beginning to reach maturity.  To determine if mowing had an effect on the size of the ten new species added to the prairie reconstruction, non-destructive measures of plant size data were collected in July of 2005.  Four of the ten species, Dalea candidum (White prairie clover), Astragalus canadensis (Canada milkvetch), Aster laevis (Smooth blue aster), and Parthenium integrifolium (Wild quinine) were selected for measurement, as they were most abundant throughout the plots.  Maximum height and stem number were recorded for up to fifteen randomly selected individual plants per plot for each of the four species.  

2005 Transects

In the summer of 2005 mowing had ceased.  To determine if mowing had been detrimental to the 23 species of native forbs added by Dave Williams in 1999 and to gain an understanding of the plant community composition, 20m x 1m transects were established in each of the plots.  All native forbs found within these transects were counted and recorded.

Grasses

To determine the effects of mowing on cool season grasses (Kentucky bluegrass and Smooth brome), we counted grass panicles in June of 2005 when cool season grasses were at their peak of flowering.  The panicles of all cool season grasses found within five, 0.25m² quadrats were counted in each of the plots.  All flowering stalks of warm season grasses found within five, 0.25m² quadrats were counted and recorded in each of the plots.  Data was collected in September of 2005 when prairie grasses were in full flower.

PART II (Addition of Conservative, Showy and Expensive Species)

Experimental Design

This experiment was a randomized block design.  A 15m x 20m area was set aside on the UNI campus prairie where there was a high density of warm season grasses.  In the fall of 2004 the area was burned in preparation for the addition of forb seed.  Twenty, 3m x 5m blocks were then established in the area.  Half of the blocks were designated for the addition of Midland shooting star (Dodecatheon meadia) while the other half was designated for Prairie phlox (Phlox pilosa).  Both species are considered to be conservative and showy as well as expensive.  Within each of the blocks four, 50cm x 225cm plots were permanently fixed and assigned to one of four treatments.

In 2005, the treatments for this experiment included two different planting times and two different pre-planting seed treatments.  The planting times included an early planting (March 12^th) or a late planting (May 23^rd).  The pre-seeding seed treatments included stratified (cold/wet treatment for four weeks) or unstratifed.  All plots were seeded at a rate of 1.3seeds/cm².  All plots were mowed throughout the growing season to keep the established vegetation at a height of 15cm.  Plots were scouted for germination until seedling emergence was detected (June 9^th 2005); at that time seedling censuses began and were conducted every two weeks until seedling numbers began to decline.  Seedlings were counted beginning in May 2006. Using a 50cm x 25cm quadrat broken into a grid of 5cm x 10cm rectangles the entire plot of phlox was counted. Due to the high number of shooting star seedlings we randomly chose five grid points from each quadrat to sample. Effectively we sampled 20% of the plot.

During 2006, a new experiment was planted using blocks reserved for this purpose.  The same two forb species were used, but treatments were modified to reflect and supplement concurrent experiments with seed predation. The treatments for the 2006 seeding include two different planting times and two different pre-planting seed treatments. The planting times include a spring planting (April 5^th, 2006) and a fall planting (October 2006). The pre-planting seed treatments include capsaicin treatment (Squirrel Away^®) or untreated. Capsaicin was applied by shaking the seeds in a plastic zip-lock bag with the manufacturer’s recommended rate of 7.9 g/kg seed.

For the 2005 planting we analyzed the data using an ANOVA looking for differences among the planting time and seed treatment. The spring 2006 planting we only used ANOVA for the differences between seed treatments.

Seed Disappearance

Results of the 2005 seedling counts indicated very poor germination in all of our experimental plots.  To uncover reasons behind such poor germination we looked at the amount of seed remaining in the soil.  We extracted 15cm x 15cm sod sections two inches thick, from all of the plots that had been seeded with Prairie phlox in March. Based on the seeding density we estimated one-hundred seeds should be recovered in the extracted sod section.  Using soil sieves, we removed as much sod as feasible and counted the remaining seeds.

RESULTS

PART I (Incremental Forb Enhancement)

Overseeding a mature prairie reconstruction with ten new species in 2003 resulted in the long-term addition of six new species to the plant community.  By summer 2005, three species were found in high abundance 0.90 plants/m2), three in moderate abundance 0.37-0.50 plants/m²), and one in low abundance (<0.2 plants/m2) within the research plots  (Table 1).  All of these species were observed flowering in 2006.  Total species diversity per square meter sampled also doubled (Carolan 2006) and this difference was visible to the casual observer.

Mowing at any interval appears to create a more positive environment for newly emerging seedlings by making certain necessary components more available. Results of destructive sampling showed that the increased availability of resources in mowed plots yielded larger seedlings of wild quinine, our most abundant forb species.  In summer 2004, wild quinine (Parthenium integrifolium) was significantly larger (p=0.005) in plots mowed either frequently or infrequently (Figure 2).  However, when less abundant species were measured nondestructively, using stem number and height, we found no difference in plant size between any of the mowing treatments for three of the four species (Astragalus canadendsis, p=0.81, Aster laevis, p=0.068, and Parthenium integrifolium p=0.85).  One of the four species, White prairie clover (Dalea candidum) was significantly (p=0.02) larger in plots that were mowed infrequently compared to plants of this species in control plots (Figure 3).

Results of the 2005 transects show that mowing did not have a detrimental effect on the existing plant community.  There was no significant (p=0.18) difference in the number of forbs found in plots that received a mowing treatment as compared to control plots (Figure 4).

Mowing increased the abundance of cool season grasses, but did not affect warm season grasses.  Mowing frequently or infrequently significantly (p<0.001) increased the occurrence of seed panicles in the cool season grasses (Table 2).  The number of seed stalks produced by the warm season grasses was not affected by mowing (p=0.97, Table 2).  

We had anticipated comparing the flowering density (percentage of forbs in flower) in mowed and unmowed plots, but this was not practical in summer 2004 due to the mowing regime.  In summer 2005, few plants flowered probably due to the two consecutive years of mowing.  However, by 2006 we did see considerable flowering of newly established species as well as forbs established in the previous experiments (figure 5). This will be quantified further in 2007.

PART II (Addition of Conservative, Showy and Expensive Species)

In the first year after seeding,  Dodecatheon meadia had 0.0% germination in replicated plots.  Phlox pilosa had nearly 1.0% germination in all plots that were unstratified, however there was 0.0% germination in stratified plots.

Results of the seed disappearance study indicate that a large amount of seed is being lost perhaps through predation by insects or small mammals.  We found a maximum of fifteen seeds in the extracted sod samples which by our estimations should have contained approximately one hundred seeds.

In year 2 of the study, sufficient germination took place to formally compare treatments.  Neither stratification nor planting time affected germination of shooting star (p>0.05; Figure 6a). All four of the treatments averaged from 211-311 seedlings/m². Prairie phlox (Figure 6b) showed a very strong negative response to stratification (p=0.002). The stratified phlox plots had seedlings ranging from 0-6 seedlings/m² compared to the unstratified ranges of 119-214 seedlings/m² (Figure 6b). The planting time for phlox showed no significant effect for either of the stratification treatments (p>0.05). Even for the best plots the germination rate was no more than 3.8% , or 577seedlings out of 15,000 seeds planted.  Overall the average germination was still under 1%.

Phlox and shooting star planted in May 2006 experienced greater first-year germination than those planted in 2005, but it was still very low; on the order of 0.3-0.6% (Figure 7).  Capsaicin treatment had a very strong positive effect on the germination of shooting star (p=0.002). The untreated seeds produced 39.8seedlings/m² compared to the 80.8seedlings/m² recorded in the capsaicin treated plots (0.3% to 0.6% germination; Figure 7). The phlox germinated at a rate of 0.2% regardless of capsaicin treatment.  Seed counts will continue in summer 2007.

DISCUSSION AND MANAGEMENT RECOMMENDATIONS

PART I (Incremental Forb Enhancement)

Thorough investigation over the last four consecutive growing seasons has allowed us to obtain answers to our research questions listed in the introduction.

Can we continue to add forb species incrementally, when there are already a few species of forbs in the grasses, or will mowing kill the existing forbs? 

Yes, it is possible to add forb species incrementally to an established stand of grasses. Visiting the research site today it is obvious that incremental addition of forbs has been successful as species that were seeded in 2003 are thriving and blooming next to species that were seeded in 1999 (Figure 5).

Is it necessary to mow after overseeding a reconstructed prairie with new forb species?

Our evidence is mixed.  Two of the four species studied showed a positive response to mowing. We feel this is a very important finding because larger plants are more likely to persist for an extended period of time and reach reproductive maturity (Figure 3). Mowing also increased cool-season grass cover but did not appear to reduce warm-season grass cover, in terms of density of flowering stems.  We expect this to be a temporary effect.

We did discover that mowing can be done less frequently than once a week and still yield the same beneficial results.  This will reduce the workload for roadside managers and make forb enhancement projects more economically feasible.

Do some forb species respond better to this technique (mowing) than others?

Yes, not all of the species included in this study did well using the burn/overseed/mow method (Table 1).  It is important to determine which species do best under these planting conditions so that valuable time and effort are not wasted on the addition of species that will consistently fail.

PART II (Addition of Conservative, Showy and Expensive Species)

Does planting time affect the success of more conservative species?

Both Phlox and Shooting Star delayed major germination until the second year, and then germinated in equal numbers with respect to planting time.  However, we can’t extend these findings to other showy, conservative species. Some species may respond very differently to different planting times and in different years depending on the weather.

Do pre-planting seed treatments affect germination in conservative species?

Depends on the species.  Phlox treated with a cold-wet stratification resulted in only a few seedlings compared to hundreds in the unstratified treatment (Figure 6). Stratification had no affect on the shooting star success.  It is well-known that stratification has extremely positive affects on other species. Clearly, we need to focus on learning more about specific germination preferences of each prairie species to have big increases in restoration success.

Are non-germinating seeds remaining in the seedbed or lost through death in the soil and predation?  What can be done to reduce seed losses?

While seeds did germinate better the second year after planting we still saw only 1% germination total in the best plots, suggesting that seed death through disease and predation is an important factor. Subsequent seed predation studies in summer 2006 suggest a very strong role for seed predation by rodents and ants—upwards of 90%, even for extremely tiny seeds such as Shooting star.

Capsaicin has the potential to reduce predation.  We observed this in the shooting star plantings as the capsaicin treated seeds consistently had over twice as many seedlings as the control (Figure 7). Although the phlox plantings did not show a consistent difference we may need to wait until 2007 to make a clear determination. Many of the seeds may have remained dormant in the soil and will germinate until next year.

Figure 1: Forb species added by Dave Williams in 1999, still thriving in 2006.

Figure 2: Wild quinine found in frequently mowed treatment (left) in 2005 compared to unmowed control (right).  Plants in mowed plots were significantly heavier (p=0.005).

Figure 3:  Effects of mowing on plant size. Composite size (stem number * maximum height) of individual plants sampled for four of the ten new species added in 2005.  White prairie clover was significantly larger in mowed plots (p=0.023).  No other species showed a mowing effect using this nondestructive sampling method.

Figure 4: Average umber of plants per m² per plot in 2005, based on 20m x 1m transect sampling.  Error bars are plus or minus one standard error.  One-way ANOVA showed that mowing did not significantly affect the number of forbs found in each plot (p=0.185).

Figure 5:  Species added to prairie in 1999 existing with species added in 2003.   Photo taken July 12, 2006.

Figure 6:  Average number of seedlings per m² for each of the four treatments sown in Spring 2005, for Shooting Star (top) and Phlox (bottom). These data were recorded in May 2006.

Figure 7:  Comparison of seedlings/m2 observed from capsaicin treated and untreated seeds. Data recorded May 2006.   Phlox showed no capsaicin effect, but Shooting star germination was doubled by the application of capsaicin.

Table 1:  List of 10 new forb species seeded into plots during second incremental planting in 2003, and their relative abundance in plots in Summer 2006.  All species were sown at a rate of 25 seeds/m² per species; differences in abundance reflect success of establishment and growth over 3 growing seasons.

Table 2: Mean (standard error) of the number of flowering stems of both the cool and warm season grasses by treatment.  Mowing significantly increased the amount of cool season grasses that flowered compared to controls.  Mowing did not significantly change the abundance of warm season grasses that flowered.