Designing Better Wildflower Mixes (Ep. 83)

Looking to create a buzzing spot for bees in your garden, you head over to the local nursery and pick up a wildflower seed mix labeled pollinator friendly. Knowing what you know, you’d expect the seeds to be from flowers that bloom at different times of the year, be local to the area, and offer a diverse selection to appeal to as many local bees as possible. But in reality, many mixes aren’t designed with such thought. Cost and a one-size-fits-all approach are undermining many wildflower mixes.  

So what should we do? Dr. Uta Müller conducted a multi-year field experiment in two very different climates to see what exactly makes a good wildflower mix. In this episode she talks us through why grasses are an unexpected problem, why it’s perfectly acceptable to have a low seeding rate to keep costs down, and the importance of matching your mix to your climate.

Dr. Uta Müller studies restoration for wild bees. She collaborates with both the Cariveau Native Bee Lab and the Williams Lab at the University of California, Davis. This is the paper we discussed.

Good to know

Uta and her team looked at the impact different wildflower mixes had on forb richness and the phenological coverage of the resulting plants. Phenological coverage of plants relates to the duration of observations (the entire blooming season), the geographic coverage (the test site), and the diversity of plant species. An area with high forb richness contributes to biodiversity and supports pollinators.

Transcript

[00:00:00.270] – Jacy

Welcome to The Bee’s Knees, a podcast wild about native bees. Wild and native bees are under threat worldwide. In each episode, we look at actionable things we can do to support these adorable little guys whose pollination work is crucial for maintaining biodiversity. I’m Jacy Meyer, and I thank you for being here. Pollinators, especially our wild bees, are facing global declines driven by habitat loss and the shrinking availability of flowers.

One of the most widespread responses has been to plant wildflower strips and restoration plots using pollinator-focused seed mixes. But here’s the catch: these mixes vary wildly in cost, composition, and performance. And we still don’t fully understand how their design choices shape the flowers that emerge. So what actually makes a pollinator seed mix work? That’s what we’re discussing today with Dr. Uta Müller. She and her team conducted a study across two very different regions of the United States: California’s dry Mediterranean climate and Minnesota’s wetter temperate grasslands. They ran a multi-year field experiment to test how seed mix parameters like species diversity, seeding density, and the inclusion of grasses influence the flowers that pollinators actually get. Uta tells us more. So one of your clearest findings was that grasses consistently reduced floral area and phenological coverage, especially in the early years.

[00:01:41.680] – Jacy

Why do grasses have such a strong competitive effect? And what does this mean for people designing pollinator plantings?

[00:01:49.200] – Uta

Yeah, first, thanks for giving me the opportunity to share our research here. Actually, we ask ourselves the same question, and to answer it, we have to look a little bit at the biology of grasses. So in general, they establish faster than wildflowers, and they can form a thick canopy on the ground, and in this way, they capture space, light, water, and nutrients from the wildflower seedlings. So these then have a harder time to get established and to flower at all, and they might even struggle to persist. So that requires land managers to regularly mow the plantings or try to suppress nutrients. So eventually that results in a higher maintenance effort. So in this way, it might be tempting to just omit the grasses in the pollinator seed mixes, but still, grasses clearly do have their benefits. Because we need to ask ourselves the questions: What are the original lost pollinator habitats that we are trying to compensate for? These are, for example, native grasslands like dry calcareous meadows or tallgrass prairie, just to put out some examples, and they contain grasses. So grasses provide multiple functions in a habitat. Beyond foraging material, bees also need nesting sites.

[00:03:06.650] – Uta

and wild bees, for example, can nest beneath the clumps of dried grass blades. Then some species have their pupae overwintering in the stalks of dried grasses. Also, we want to support non-bee pollinators, so for example butterflies, and these ones might need grasses for their caterpillars, for example as food or as shelter. And very ideally, pollinator plantings should also provide a habitat for as many other species as possible, from insects to birds or even small mammals, who also need grasses in multiple ways. Then, grasses definitely have different species, and these species differ in their competitiveness. So one way to cope with the problem of grasses would be to select grass species that are less dominant and use wildflower species that are more competitive.

[00:03:55.430] – Jacy

So your study showed that increasing the forb richness boosted both floral abundance and the season-long bloom in many cases. What is it about the diverse forb mixes that make them so effective for supporting pollinators over time?

[00:04:10.170] – Uta

Yeah, that’s a good question. I guess everyone can picture this colorful flowering patch that pleases us so well as pollinators, but it’s certainly not just a matter of esthetics, of course. So the high forb diversity provides an insurance in the case of failure of some species. Because there are always wildflower species that might not establish, that might disappear quickly, particularly in multiannual applications, or that only provide a very low amount of flowers. Yeah, we’ve clearly seen that in our mixes. We tested all of the seeding material in separate mono-species plots, so that was to make sure that all of the species really come up, which they did, but still that didn’t mean that they were also thriving in mixes. So that’s why we need to keep in mind that the flowering community of a planting might really deviate from the original composition of the mix. Then it’s important that pollinator habitats provide consistent nectar and pollen supply over the whole season, because there are bees with flying periods rather early in the year, some in the middle season, and some might fly relatively late. So for all of the different flying periods, we need to provide enough flowers.

[00:05:26.500] – Uta

Plantings should ideally last over multiple years just to make sure that the next generation of wild bees is supported. Everyone who knows a little bit about native bee biology is aware that native bees prefer different flowers. There are some bees that are highly specialized, so that means they only forage on certain flowers. For example, there are bumblebees with very long tongues that are adapted to flowers with very deep corollas. Then also in general, a high flower diversity also means that we can support a high native bee diversity. That is quite important because very often plantings will support rather, rather generalist species. That’s what many studies show. Specialists are still relatively neglected. Then furthermore, flowers differ in their nutritional value. So the nectar, the nectar amount, sugar concentration, or if nectar is provided at all might differ depending on the flower, and also native bees have clear preferences of pollen from different flower species. So to wrap it up as a takeaway message, we can say that a higher crop diversity basically provides an insurance of plant species who tolerate stress in different ways, bloom at different times of the season, and attract different species of pollinators.

[00:06:47.400] – Jacy

So one surprising result was that seeding density had little effect on floral area when the forb-grass ratios were held constant. How should this reshape how land managers think about cost-effect seed mix design, as you were just mentioning?

[00:07:04.530] – Uta

Yes, that’s important because costs are really a crucial point in pollinator habitat planting because wildflower seeds are expensive, and the financial support, for example subsidies, are lacking. That can mean high costs, and these can clearly impose a barrier to land managers that are interested. But luckily, we can say that there are options to diminish costs and still have effective applications. So there are different approaches. One would be to manipulate the seeding rate, so that means just using less seeding material. Another way would be to reduce the seed diversity because different species of wildflowers have different prices, or we could also just include grasses because those seeds are less expensive. And that is basically what we tested in our study. So what did we find out? We had two experiments. One was testing a rather short-term wildflower application in a California environment, and the other was looking into a long-term grassland restoration site in Minnesota. And very interestingly, for both sites, our results suggest that we can reduce the potential cost by lowering the seeding density. Because, and that was our criteria, that did not result in a diminished floral resource return and also not in a diminished phenological coverage.

[00:08:22.170] – Uta

So we would recommend to start with a low-density seeding. However, if the plantings turn out to lose their floral coverage over the years and reseeding might be necessary, we would also ask the land managers to rather reseed low density again instead of starting with a high-density seeding event already and from the beginning on. On the other hand, lower species diversity in order to reduce the cost is something we would not recommend because we showed that it clearly compromises the phenological coverage of our planting.

[00:08:55.080] – Jacy

So let’s talk about those two sites that you studied, California and Minnesota, and they behaved very differently over the three years that you conducted this study, especially in how quickly the floral resources developed in each location. What do these regional contrasts tell us about tailoring seed mixes to climate and restoration goals?

[00:09:18.260] – Uta

That’s why we initially chose two sites in very different environments, because the ideal seed mix for a location depends on a lot of different things. That might sound trivial, but it’s actually really important for good practice. Yeah, so when choosing a seed mix, it requires to know the restoration goals. Are we looking at small and short-term habitat enhancements such as wildflower strips in agricultural environments, or are we targeting long-term restoration of more natural habitats such as diverse meadows? Because the chosen plants in the mixes might clearly differ. So in short-term applications, we would use a lot of annual species because they provide a lot of flowers already in the first year. But unfortunately, often then they might decline relatively quickly. So from the second year on, already we might lose flower coverage, and these plantings only persist for longer if they manage to reseed. That actually turned out to be a little difficult. That’s what we’ve seen in our California experiment. So a solution is to include perennial species in the mixes because they are way more sustainable. And we have seen that at our Minnesota sites. Here we focused on perennials, and compared to California, we hardly got flowers in the first year, and then flowering started in the second year.

[00:10:42.190] – Uta

We didn’t have a big burst of flowers, but we had way more stability from the second year on. And actually, that aligns quite well with other studies, so we can safely conclude that the proportion of perennials in a mixed kale matters. And then we want the mixes to be tailored to the local environment. So that means we want to use plants that are adapted to the local climate, to the local soil type, moisture level, and of course to the different pollinator communities. So as an example from our study, we had two sites, the Mediterranean and the temperate climate. And we’ve clearly seen profound differences in the vegetation period, for example. So in the Mediterranean, we have the spring bloom with a big outburst of flowers earlier in the season, but then usually it’s followed by a drought with very low amounts of flowers. And in the temperate climate, however, we have freezing temperatures in the winter and the flowering starts much later in the season, but then it’s extended over a longer spring and summer period with no peak bloom. So the chosen plants in the mix should ideally reflect the local wildflower community, and that’s why we would strongly advise against mixes that use a lot of non-native or ornamental plants.

[00:11:54.580] – Uta

And last but not least, what’s often not kept in mind is that the seed material should be sourced locally. So that is the best way to get adapted species to the specific environments.

[00:12:05.170] – Jacy

So I want to go back to something you mentioned previously about how floral resources decline over time and that the grass dominance increases. Knowing that, what long-term management actions, and you kind of touched on this a bit, receding, disturbance, selective mowing might be necessary to keep pollinator plantings functioning?

[00:12:27.440] – Uta

Yes, good question. The management is definitely something that should not be neglected, and we’ve clearly seen that in our experiments. We had them running for 3 consecutive years, and in both sites we could observe that the abundance of flowers was not sustainable in the long term. So one management action would be to resow the plantings every once in a while. because the natural reseeding might not seem to be sufficient. Then another action we would recommend is to suppress the grass vigor. So that is best done by timed disturbances, such as for example mowing, grazing, or depending on the climate, even burning, and that gives the wildflowers new opportunities to resprout. So it’s important here to find the right timing and frequency so that it doesn’t negatively affect the pollinators. So for example, mowing should be tailored to the individual site, so that means it should be done a bit more selectively, so not cutting all of the area of the plantings at the same time. Also avoid cutting in the main flowering period, of course, and then consider the activity periods of the occurring pollinators. The other option, grazing, is a little more subtle, but it creates more structure and microhabitats.

[00:13:39.520] – Uta

so it might eventually leave more refuges for the pollinators. Especially in the Mediterranean, when we look at our sites, it’s worth finding out if irrigation is a part of the plan. So, for example, if the intention is to keep the intervention low and additionally also to consider what future climate trends are we expecting, in this climate it’s clearly advisable to use local drought-adapted species.

[00:14:03.280] – Jacy

So, in all your time working with bees and in biodiversity, has there been a discovery or some other part of your work that has given you hope or made you feel like you were making a difference?

[00:14:14.900] – Uta

Oh, definitely. It’s making me really hopeful to see more and more native wildflower plantings everywhere in public areas, in urban environments, in gardens, and even in agricultural areas. And I really hope that this is even expanded and also that the applications are done properly. Because it’s really crucial that the right plants are chosen and also that these applications are taken care for, for the long term, because still a lot of plantings are still not lasting long, but that is what we really need and what is essential to sustainably support native bees.

[00:14:49.820] – Jacy

So what does all of this mean for the people actually putting seeds in the ground? Uta broke it down perfectly. First, wildflower seeds are expensive. Adjusting seeding density and choosing the right level of plant species richness can make mixes both more affordable and more effective. Second, climate and landscape context matter. In California’s Mediterranean system, mixes delivered quick wins and strong spring blooms. In Minnesota’s temperate grasslands, floral area built more slowly, peaking in summer. But grasses also became more competitive over time. That grass dominance will need to be managed. And finally, seed mixes aren’t one-size-fits-all. They can be tailored for short-term agricultural enhancements, long-term natural area restoration, or anything in between. The key is matching the design of the mix to the ecological setting and the goals of the project. Thanks so much to Ute for the terrific conversation. Learn more about this study and Uta’s work on the website, thebeesknees.website. If you know someone whose landscape would benefit from some wildflower intelligence, please share this episode. Thank you so much for listening, and until next time, keep planting.