Bocking comfrey orchard understory cover (Site specific strategies; video included)

Article written by Madeline Wimmer, UMN Extension Fruit Production Educator

Two photos: left, showing up close of a comfrey plant and its purple drooping flowers. And, right, a photo of an apple orchard row with plants, including comfrey, within its understory.

Images: Flowering Bocking 14 comfrey at Alternative Roots Farm (left) and an apple tree row with Bocking 14 comfrey before it collapses into a mat (right, photo by John Knisley of Alternative Roots Farm, New Ulm, MN).

Introduction

Conventional herbicides are commonly used to manage unwanted vegetation in orchard understories in conventional management. Because these products come with their own drawbacks and limitations, and are not allowed for USDA Organic production, organic producers, and orchards looking to cut down on herbicide use may explore alternatives.

While a few herbicides are listed in the Organic Materials Review Institute (mostly contact herbicides) many non-chemical ground management options can also be considered. Practices can include mowing, mulching, cultivation, strategic livestock integration, and establishment of an understory ground cover.

This topic and a number of these practices were overviewed in a previous webinar from the UMN Fruit Program:

Organic Weed Management: Organic Commercial Fruit Production Strategies for the Midwest

Many non-chemical ground management practices go beyond managing unwanted vegetation by contributing to soil organic matter as mulches and ground covers decompose after mowing, and alter other factors like soil temperature and moisture retention. Not all methods are the same, as practices like tillage require specialized equipment, while establishing a cover crop may take time, attention, and added mowing, depending on the crop.

Establishing a tree-row understory crop can help with weed management and be tailored to individual site goals. Many cover crop species are densely seeded with the intention to cover as much ground as quickly as possible in order to outgrow competing weeds, while some legume crops, like white clover, can also fix some amount of nitrogen. In contrast, the crop this article is focusing on today, sterile comfrey, works by shading surrounding plants as it senesces throughout the season.

Bocking comfrey (Symphytum x uplandicum)

A photo showing the ground under a trellised apple tree row with a senescing comfrey plant with leaves collapsed onto the ground.

Image: Bocking comfrey collapses after flowering as seen in this photo taken at Atoms to Apples Orchard in Mt. Horeb, WI. The grower in this orchard planted comfrey at the end of some of the high density, trellised apple orchard rows.

Comfrey (Symphytum spp.) is an herbaceous perennial that thrives in temperate climates, growing well in full sun to partial shade environments. There are many different species, but not all are recommended as a ground cover in agricultural settings. For example, common comfrey (Symphytum officianale) is often avoided because of the number of seeds it disperses, making it an aggressive spreader in Minnesota.

Common comfrey's growth habit is much different than the Bocking series (Symphytum x uplandicum) sometimes referred to as Russian comfrey, and includes varieties like Bocking 4 and Bocking 14. Bocking comfrey is sterile and does not produce seeds. It grows in a clumped fashion and only spreads vegetatively when its roots are disturbed through cultivation or intentional cuttings, which means it can be propagated asexually through root cuttings.

Under trees, it can grow over three feet tall before it flowers and naturally collapses, at which point, more shoots are regenerated from its base. When the vegetation collapses after flowering, it creates a thick mat that blocks out light and helps to suppress weeds.

As the aboveground vegetation breaks down over time, it also contributes organic matter to the soil surface. Because it flowers for an extended period during the growing season, Bocking comfrey attracts pollinators, which is something to be aware of when planning insecticide applications, for example.

Prairie Ally is a food forest located in Luverne, Minnesota, that grows Bocking 14 under some of their fruit trees, and Alternative Roots Farm (located in New Ulm), which grows both trellised and free-standing apple trees and utilizes Bocking 4 on a larger scale. Because of the large area covered after senescing, each Bocking comfrey was planted once every few feet at Alternative Roots. And, it was quite apparent where Bocking comfrey had been planted, even earlier in the growing season, because of the bare ground around it. Alternative Roots notes they have tried out a number of different plants as an understory crop and have continued to use comfrey as a prominent choice for understory management.

In certain permaculture literature, Bocking comfrey is often described as a 'dynamic accumulator' due to its deep taproot. Scientific studies confirming its ability to mine subsoil nutrients compared to other perennials are limited, and it’s important to keep in mind that many plants accumulate nutrients. Ultimately no nutrients will be added to the soil that don’t already exist.

Comfrey may not work as a plant for every orchard, and there is still some consideration about the moisture it may contribute around the tree trunk. It has its advantages for those who are interested, including some amount of nutrient cycling when it decomposes, and its ability to shade out weeds.

To see more video footage and learn more directly about Alternative Roots Farm’s use of comfrey, check out this interview with John Knisely, orchard manager and founder:

Video: Comfrey ground cover in an apple orchard