Considering Root Diversity

Baptisia australis flower

When planning projects we are often considering diversity: the diversity of the animals who will be using them, pollinators, beneficial insects, birds, mammals; the diversity of seasonal interest: flowering time, showy fruits, good fall color, winter structure; the diversity of texture, shape and size. But have you considered the diversity of the roots of plants you are putting in?

We are thinking about roots all the time.

Why consider root diversity?

Plant roots come in, basically, two styles: Taproots – think carrots or dandelions, one large root relatively unbranched – and fibrous roots – think many of the other weeds you may pull out of the garden with lots of branched roots and no obvious dominant root.

Most roots will hold soil, clean water, foster microorganisms, break up soils, draw and shed nutrients and add organic matter as they decompose after death. And you have likely seen this illustration showing the incredible root systems of native prairie plants. Why consider the diversity of root types when planning your projects? Because they offer different benefits and behaviors in the landscape.

The many branches of a fibrous root system can help with erosion control, holding soils in place in disturbed areas. Typically residing closer to the soil surface and found in monocots (like grasses and plants in the lily family), they are designed for absorption, capturing the quick moving water-bound nutrients like nitrogen as quickly as possible through their net of roots. They are also generally wide-spreading. Though they are not taproots, especially in native grasses, they may be found growing deep in the soil profile. Often these plants spread thickly forming mats or great colonies.

Asclepias tuberosa

Taproots, typically found in dicots and conifers, can mine nutrients from deeper in the soil profile. They are designed for support and storage. They evolved to help a plant survive droughty conditions and seasonal climate changes. These large deep roots are also great for breaking up subsoils that may be causing drainage problems on a site. This why it is popular to use large forage radishes as cover crops in organic agricultural production. Also known as tillage radishes, the vegetable’s taproots not only break up compacted soils but as they decompose they leave large holes in the soil allowing space for water and nutrients to settle and percolate and reducing the need for cultivation. Taprooted plants are often clay soil tolerant because of their ability to break through the soil.

Research has shown that root type also influences the type of fungal associations in the soil network and it is possible for the fungi present in the soil to influence roots. Having greater diversity in roots present in your plantings will equal greater soil food web diversity and soil structure more suitable for a wider variety of plants to grow. Thinking about diversity beyond the above ground features of a plant is a large part of Thomas Rainer and Claudia West’s Planting in a Post-Wild World planting schemes. In their process they select plants based in part on the function they will serve in the landscape.

What plants should you consider for root diversity in your landscape projects?

Taprooted Plants

These are the plants you probably dread transplanting. They tend to form clumps, rarely spreading into large colonies through their root systems, relying more on seed dispersal for expanding their range.

Baptisias – Have you ever tried to transplant an established Baptisia? Trying to relocate or divide just one requires a team of people, a wide variety of tools usually including a saw or something with an engine and a goal of sharing a six-pack at the end of the ordeal (Or is this just us?) Those Baptisias have a substantial taproot system regardless of species.

Liatris – Not only do they have substantial roots but they are great pollinator plants and make great cut flowers too!

Asclepias – Drought tolerance is a theme among taprooted plants. These deep root allow plants to access water located further down in the soil profile. Their ability to mine water and nutrients from below allows them to thrive in seemingly nutrient poor and/or droughty soils. Asclepias are no exception to this.

AmsoniasBluestars provide ice blue spring flowers and often a great fall color. Check out this article on the many species and landscape uses for Amsonias.

Eryngium – Called rattlesnake masters because the plant was thought to have snake bite curing properties. These plant looks drought tolerant and is reminiscent of desert plants featuring yucca-like foliage and spines along the leaves.

Fibrous Rooted Plants

Monocots such as grasses and plants in the lily family tend to have fibrous roots. Interestingly, if a taprooted plant is not experiencing drought or nutrient stress it may change its root architecture to be more like a fibrous root system.

Grasses – Any of the native prairie grasses would add fibrous root structure and its erosion preventing benefits to a landscape project.

AlliumsOnions, though often associated with the edible bulb, also come in fibrous root types – think of the chives in your herb garden.

Allium cernum, Nodding Onion

Iris Family PlantsBlue-eyed Grass, Sisyrinchium angustifolium, may be small but do not underestimate its coarsely fibrous root system.

Photo: K V Salisbury
Blue eyed grass, Sisyrinchium angustifolia, along a trail with Potentilla

Spiderworts – named because they resemble another plant that was supposed to cure spider bites, Tradescantias have a thick fleshy fibrous root system.

Tradescatia ohioensis ‘Mrs. Loewer’

Of course there are many more taprooted and fibrous rooted plants as options for your projects, this is just a sampling to get you thinking about adding root system diversity to your list of considerations when choosing plants for your next project.

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