Posted: February 5, 2016
Project Goal: Evaluate native plant species and their cultivars for their attractiveness to pollinators and their suitability for homeowner and agricultural use.
Not all plants are equal in their ability to support pollinators with nectar and pollen. Some cultivars and modern hybrids may actually lack nectar, or be so complex that pollinators cannot utilize the plant. The results of this trial provide information about which plants are the best at supporting pollinators. Some information on the flowering of these plants is also included.
Why Evaluate Pollinator Friendly Plants?
- Insect pollinators are responsible for 1 of every 3 bites of food that we eat.
- Pollinators are increasingly challenged by lack of good nutrition, places to nest and pesticides in today's agricultural, suburban and urban landscapes. Research to determine the plants most beneficial to pollinators will help homeowners and farmers make better choices when purchasing plants for their properties
Project History:
August 29, 2011 - Penn State Master Gardeners planted 4500 plant plugs representing 86 species and cultivars of native plants in approximately 1/3 acre at the Penn State Southeast Agricultural Research and Extension Center. Plants were chosen that would provide a succession of bloom from spring through fall and that were known to attract pollinators. One week later, tropical storm Lee left the plot under several feet of water for several days. Most of the plants survived, attesting to the hardiness of these species.
2012 to 2015 - Two teams of staff and Master Gardeners evaluated plants for their insect visitation and for their vigor and blooming. Monitoring was done weekly. Monitoring was done by observation and collection. Collected insects were evaluated at PDA during the 2012-2013 growing seasons.
What Did We Learn?
The data that was collected each year was summarized and presented as annual updates, so the objective of this fact sheet is to provide summary information over the 3 years of the project.
Although not a specific objective of the project, Figure 1 provides a suggestion of plants that pollinators find attractive during the growing season. The plants selected flower sequentially throughout the year beginning with Packera aurea and ending with Symphyotrichum laeve 'Bluebird', and were selected because these plants have fairly high pollinator visits. It is not suggested that all of these are needed or that other plants not listed would not be appropriate. Of the plants tested these seemed to have many pollinator visits. The figure also provides some information on the duration of bloom. For example Eutrochium fistulosum 'Bartered Bride' flowered about 4 weeks while Pycnanthemum muticum was in flower for about 15 weeks. The data in Figure 1 was specific for the 2014 flowering season.
In 2012 a total of 72 species of bees, wasps, syrphid and tachinid flies were identified from these plots (data not included). The most common insect visitor was the tachinid fly, which is a generalist parasitoid of moth larvae. This fly was mostly attracted to species with small flowers such as Pycnanthemum muticum. The bumble bees and small-sized bees were the most common bees (Ellis, 2013).
The plant that was most attractive to pollinators was Pycnanthemun muticum (Table 1). This plant was most attractive each year and was most attractive for pollinators when pollinator visits were summed over the 3 years. When it was in flower the plant seemed alive with activity. The number 2 plant for total visits was Solidago rigida; however, it was second only in 2014. In 2012 Solidago nemoralis and Eupatorium hyssopifolium had more visits than S. rigida, but when totaled over the three years S. rigida was number 2. At this point there is no explanation for year to year variations in visits per plant. There are a couple of factors that may play a role. Each year the plot was a year older and some plants flourished and grew very well. Other plants grew but did not flourish. Data was collected weekly so weather as well as flowering season may have had an effect on what the pollinators found attractive on any one day. Even with this limitation, these data will provide an indication of plants that pollinators find attractive. For plants that will be attractive to pollinators and be colorful garden plants select plants such as Solidago, Liatris, Asclepias, Monarda and Symphyotrichum.
The second 10 plants most visited by pollinators are all colorful garden plants and had somewhat similar number of pollinator visits (Table 2). The difference in the number of visits between number 11 and 20 is relatively small suggesting that all of these plants are attractive to pollinators.
An issue of concern is whether or not cultivars, which are generally selected for horticultural benefits (larger flowers, different color), are as attractive to pollinators as are the species. In this work there were 14 direct comparisons and about 50% of the time the species was better than the cultivar (Table 3). For selected comparisons the species (ie. Monarda fistulosa) was substantially better than the cultivar (ie. Monarda 'Claire Grace'). While in other comparisons the cultivar (ie. Symphyotrichum 'October Skies') was substantially better than the species (ie. Symphyotrichum oblongifolium). It appears that it is not possible to generalize that the cultivar is better than or poorer than the species.
At the end of the 2014 year data were collected on plant height and width, plot coverage, and promiscuity (Table 4). The data on plant height and spread only provide information on how plants were growing in these plots and may not be an indication of how those plants will perform in other garden settings. The coverage rating may provide an indication of why a specific plant may not have had many pollinators. For example Eurybia spectabilis did not show up on the list of plants frequented by pollinators perhaps because the coverage was poor (0-30%) rather than pollinators not preferring that plant. There were a few other plants that did not have good coverage, but most did have good coverage. One aspect of a plant in the garden is whether or not seeds will permit the plants to spread around the garden. This process is described as promiscuity. Solidago nemoralis was extremely promiscuous with a rating of 5. No other plant in this trial was as promiscuous. There were some plants such as Eryngium, sunflower, Penstemon, and aster that spread, but at a rate that was manageable by removal of seedlings during weeding.
When evaluating pollinators it is important to look at total number of insects, but it is also important to evaluate the diversity of the pollinators (Table 5). Solidago rigida and Pycnanthemum muticum were tied for the greatest diversity of pollinators visiting. These 2 species were visited by a greater diversity than the rest of the plants followed by Eupatorium hyssopifolium with 15.3 different pollinators. The next and largest number of plants were visited by 12 to 14 different pollinators. Plant flower size and inflorescence type was highly associated with the insect diversity. Plants with large compound inflorescences of tiny flowers attracted significantly more types of insect categories than other flower types (Ellis, 2013).
Literature Cited
Ellis, K. 2013. Identifying and Promoting Pollinator-Rewarding Herbaceous Perennial Plant Species. Final Report to Pennsylvania Department of Agricuture. 16pp.
Project was funded by a grant from the Pennsylvania Department of Agriculture
