Purple Tansy: Pollinator power player & Cover crop

When I was an undergraduate, I studied abroad for six months in Stuttgart, Germany. While there at the University of Hohenheim, my friends and I would explore the Swabian countryside any chance we could get.  I was struck by a sea of purple created by what's commonly called "purple tansy" or phacelia in a once harvested cabbage field.

This flowering cover crop buzzed with buff tailed bumblebees (Bombus terrestris). These creatures were all trying to get the last bit of energy before winter arrived, and the plant happily obliged. Apt, since the German name for this plant was “bienenfreund”, or “bee’s friend” in English.

I knew it as lacy phacelia (Phacelia tanacetifolia). Despite the Germanic nickname “bienenfreund,” phacelia evolved in the dry southwest United States, not in the moist woodlands of Swabia.

Phacelia flowers. Note the "fiddlehead" like structure. Photo: AnemoneProjectors, commons.wikimedia.org

What is Phacelia?

Also known as purple tansy or scorpion-weed, phacelia is related to borage (Borago officinalis), this annual is known for its long bloom time of 6 to 8 weeks and its highly attractive, unique flower.

• Flowers: The flowering stalk has a structure resembling a fiddlehead or scorpion’s tail at its end. Over the course of the summer, multiple flowers slowly blossom as this “tail” stretches out. 
• Leaves: Carrot-like, with a lacy appearance and a coating of hairs all over the plant.
• Pollinator appeal: Rich with nectar, phacelia can serve as an energy resource for pollinators, primarily bees and wasps. It does produce some pollen, important for growth, but to a much lesser extent. 

Not surprisingly, phacelia is primarily grown for its ability to attract beneficial insects. You may also hear it classified as an “insectary plant.”

Why a 'cover crop' for your garden?

Cover crops form a living mulch in gardens because they grow thickly among each other. They help reduce soil splash and erosion, and keep weeds in check.

Cover crops are “green manures”--that is, when a gardener turns them into the soil, they provide organic matter and nutrients. In established vegetable or flower gardens, plant a green manure early in the season to improve the soil. After you turn it under, plant warm-season vegetables, bedding plants or container-grown perennials.

When to plant phacelia

Because phacelia is extremely susceptible to cold, it is a great low maintenance cover crop in the Midwest, as most winters adequately kill both the plant and its seeds. Due to its sensitivity to cold, I recommend sowing in early to mid-summer, i.e. end of May to mid-June.

Phacelia needs to be planted only about ¼ inch deep, preferably in coarser soils that are drained. Occasional watering could be useful if dry conditions persist during seeding but hates having “wet feet” as it starts growing.

How does phacelia compare to buckwheat as a cover crop?

Buckwheat reseeding itself, which can be weedy. Photo: Rasbak, commons.wikimedia.org

Another plant that fits a similar cover crop niche to phacelia is common buckwheat (Fagopyrum esculentum). Garden catalogs may have one or both species listed, and it is important to know some key differences between them to have the most success. 

I have personally grown both, and much of what I have read has matched my experience in the field.

Similarities

1. Both plants like drained soil and can grow in poor fertility soils. Neither like heavy clay nor ponding water.
2. Regarding soil fertility, buckwheat’s roots are commonly thought to help release additional phosphorus from mineral sources. The exact amount is up for debate, and likely depends on pH, weather, and soil characteristics. There is much we still do not know. Yet, there is also some evidence that phacelia can access this phosphorus, too. 
3. Both buckwheat and phacelia do fine in preventing nitrogen fertilizer from being lost by leaching, at least in vegetable systems. But other cover crops (such as grasses) do a better job of this than either buckwheat or phacelia. Neither plant “fixes” or adds nitrogen into the soil.

White flowers of buckwheat. Photo: Rasbak, commons.wikimedia.org

Differences

1. Buckwheat species are better than phacelia in suppressing weeds, mostly due to their quick growth right out of the gate. In my experience, buckwheat was much more competitive with weedy foes such as purslane (Portulaca oleracea) and chickweed (Stellaria media).
2. Being native to the arid southwest US, phacelia is more drought tolerant than buckwheat, which originated in the mild climates of southeast China.
3. Although both plants are great insectary cover crops. A British study found bees and wasps preferred phacelia over buckwheat, though both scored well. This was supported by at least one on-farm trial in the US. However, insects are incredibly diverse, and depending on the particular species, some may prefer buckwheat.
4. Buckwheat has more potential in the upper Midwest for becoming a weed than phacelia. Many extension services recommend terminating buckwheat about a week after flowering, but this can hamper pollination benefits compared to phacelia. 
5. If you have more of an acidic soil, buckwheat will perform better than phacelia. Buckwheat can grow in a pH range of 5.0 to 8.0. However, if you are a bit more alkaline—say pH 6.5 to 8.5—phacelia is more at home.
6.  

Final thoughts

As with any cover crop, make sure you know what your goals are for the garden. Setting goals can really help you narrow down what species you want. Remember, it does not have to be an either/or with buckwheat or phacelia! Feel free to experiment and see which one works for you. 

Quite frankly, any additional resources for pollinators and other beneficial insects are sorely needed in Minnesota. 

So next time you are growing red cabbage for your blaukraut (or rotkraut), see if you can bring another color and buzz to the edges of the garden—either the creamy white of buckwheat or the lavender purple of phacelia.

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Resources

• Clark, A. (Ed.) “Buckwheat”. Managing cover crops profitably. 2008. Diane Publishing. 
• MacKenzie, J. “Cover crops and green manures in home gardens.” 2018. University of Minnesota Extension. 
• North Carolina State Extension. “Phacelia tanacetifolia.” 2020. North Carolina Extension Gardener Plant Toolbox. 
• Smither-Kopperl, M. “Plant Guide for Lacy Phacelia (Phacelia tanacetifolia).” 2018. USDA-Natural Resources Conservation Service, Lockeford Plant Materials Center, Lockeford, CA 95237.
• Sullivan, C. “Buckwheat makes a good summer cover crop for gardens.” 2014. Oregon State University Extension. 

UMN Extension web pages:
Cover crops and green manures in home gardens

Cover Crop Options

Using Cover Crops: Species Selection for Gardens