DOT Project Number:  90-00-LRTF-405

Fiscal Year:  2004

Award:  $20,592.00

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

Other Project Participants:  Amy Carolan, Department of Biology, University of Northern Iowa

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. In brief, this study has found that:

            • It is possible to add wildflower diversity gradually, a few species at a time;

            • Mowing is required for the first overseeding, but perhaps not the second;

            • Mowing every 3 weeks works as well or better than weekly mowing;

            • Mowing tends to produce larger plants that can flower more readily;

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

            • Prairie phlox germinates best in early spring, without any seed stratification;

            • Shooting star may not germinate at all the first year; and

            • New seedings into burned sod may be eaten by mice and birds.

Ongoing studies will determine whether germination of showy species can be improved by overwhelming seed predators (mice, birds) with extra food at the time of seeding.

We have described our burn-overseed-mow techniques at the North American Prairie Conference (Madison, Wisconsin), the Iowa Prairie Conference (Cedar Rapids), and the Iowa Academy of Science (Des Moines).  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 hope to work with Buchannan County to plant a roadside demonstration, which should be ready by summer 2007.

INTRODUCTION

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.  As the third growing season comes to and end, we have learned the answers to several of our research questions:

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.

Light Intensity and Sod Density

To determine if mowing increased the amount of available resources for newly emerging seedlings, measurements of light intensity and sod density were obtained. 

Light intensity was measured using a Licor quantum light sensor.  All Licor readings were taken at approximately solar noon on a clear day.  Three measurements were taken at each of two randomly located sites within replicate plots. Care was taken to avoid trampled areas. Readings were collected above the plant canopy, 20 cm, and 2.5 cm above ground level.

To determine if mowing affected available resources below ground, samples were obtained to show how mowing effected sod density.  In mid-July of the first growing season, the crowns of two of the most abundant previously established forbs, Solidago rigida and Ratibida pinnata, were extracted along with crowns of bulk warm season grasses (any species of warm season grass).  Random coordinates were used to select three individual plants of S. rigida, R. pinnata, and unidentified warm season grasses in each plot.  Samples were extracted using a 5cm diameter bulb planter.  Samples were then cleaned to remove mineral soil and fine roots.  Shoots were counted and each sample was trimmed to 5 cm above and below ground; this was identified as the “crown”.  Crown dry mass was analyzed using multiple regression with shoot number as a covariate.

Plant Size

In the fall of 2004 the most abundant of the ten species added in 2003, Parthenium integrifolium (Wild quinine), was destructively sampled to determine if mowing increased plant size in terms of above and below ground biomass.  In each plot, four individuals were randomly selected and extracted using a 5-cm diameter bulb planter.  Each sample was washed to remove mineral soil and fine roots.  The cleaned samples were then placed in a drying oven until reaching a constant mass.  When dry, the samples were weighed.   

During the third growing season the seedlings that had emerged in 2003 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.

To determine if mowing suppressed the growth of warm season grasses 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 2002 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.

Treatments  

The treatments for this experiment include 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) at that time seedling censuses began and were conducted every two weeks until seedling numbers began to decline. 

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)

Mowing at any interval appears to create a more positive environment for newly emerging seedlings by making certain necessary components more available.  Mowing either frequently or infrequently significantly (p<0.05) increased the amount of available light at ground level compared to the unmowed controls.  Results of the sod density analysis also show that mowing at any interval significantly decreases the crown size of more aggressive, existing forb species (Figure 2) (Stiff goldenrod, p<0.001 and Grey headed coneflower, p=0.046).  Mowing did not have the same effect on the bulk warm season grasses; there was no difference between the crown masses of the warm season grasses among any of the treatments (p=0.33). 

Results of the plant size study show that the increased availability of resources in mowed plots yielded larger seedlings.  Wild quinine (Parthenium integrifolium) was significantly larger (p=0.005) in plots mowed either frequently or infrequently (Figure 4).  We found (stem number and height) no difference in stem number and height 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 4).

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 5).

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).             

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

Preliminary results of this study indicate that the two species selected for the second part of this experiment do not germinate well in the first growing season using typical planting methods.  Dodecatheon meadia had 0.0% germination in each replicated plot.  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 extrated sod samples which by our estimations should have contained approximately one hundred seeds.

DISCUSSION/CONCLUSIONS

PART I (Incremental Forb Enhancement)

Thorough investigation over the last three 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.  Our data has shown that mowing does reduce the vigor of established forbs such as Grey-headed coneflower and Stiff goldenrod but it does not cause mortality.  By the time mowing ceased, there were still as many individuals of these species in plots that had been frequently mowed as compared to control plots.  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 next to species that were seeded in 1999 (Figure 6).

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

Yes, while mowing slowed the growth of existing forbs from the 1999 seeding, the new germinantes that resulted from the second incremental seeding grew larger as a result of mowing.  While not all of the ten  new species added responded this way, 50% of the species involved in this part of the study were significantly larger in mowed plots as compared to controls.  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 4).

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

Yes, our data shows that mowing can be done less frequently than once a week and still yield the same beneficial results.  This is a major finding as it 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.  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?

Yes, our preliminary data has shown that Prairie phlox germinates best when seeded in early spring (March).

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

Yes, preliminary data analysis has shown that Prairie phlox germinates best when it is not subjected to any form of artificial stratification.

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

Our preliminary data suggests that seeds are disappearing.  We have hypothesized that this may be due to seed predation by insects or small mammals.  We plan to investigate our hypothesis in the spring in of 2006.

Table 1: List of 10 new forb species seeded into plots during second incremental planting in 2003.  Seeds were sown at a rate of 25 seeds/m² per species.

Figure 1: Forb species added by Dave Williams in 1999, still thriving in 2005.

Figure 2: Crowns of Grey-headed coneflower.  Sample on the left was extracted from a plot that was frequently mowed.  Sample on the right was from an unmowed plot.

Figure 3: Wild quinine found in frequently mowed treatment (left) compared to unmowed control (right).  Plants in mowed plots were significantly larger (p=0.005).

Figure 4: Effects of Mowing on Plant Size: Size (stem number multiplied by maximum height) of individual plants sampled for four of the ten new species added in 2005.  White prairie clover was significantly larger when found in mowed plots as compared to the unmowed controls (p=0.023).

Figure 5:  2005 Transects: Number of plants per m² per plot.  Mowing at any interval did not significantly reduce the number of forbs found in each plot (p=0.185).

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.

Figure 6: Species added to prairie in 1999 (Grey-headed coneflower) existing with species added in 2003 (Canada milkvetch).