I recently visited a splendid Bay-Wise demonstration site at Baltimore’s Cylburn Arboretum with Maryland Extension’s Native Plant Specialist Lisa Kuder. We dropped in on a workshop with Cylburn staff about dividing perennial plants and visited the Arboretum’s rain garden, brilliant with colorful fall native plants.

Along the way, Lisa and I discussed strategies that can be practiced to expand herbaceous native plant coverage, as we admired the Arboretum garden grounds and mansion. We noticed right away that the gardeners wisely left the post-bloom common milkweed (Asclepias syriaca) standing in all their wilted majesty throughout the otherwise manicured gardens as the seed pods developed and cracked open to release seed.

To seed or not to seed?

Cylburn horticulturalists are allowing the milkweed to go to seed and expand by putting out runners and sprouting new growth along the edges of their otherwise traditionally immaculate garden beds. Garden Educator Ron Roberto explained that they prefer to allow the milkweed to spread throughout the gardens by seed or rhizomally, by sending out runners, rather than dividing and transplanting (although I have personally had some luck transplanting “rescue” milkweeds from dense patches a few times).

In days past, people may have viewed common milkweed as a messy weed to be eliminated from such stately grounds, and some still may. Now its importance to the rapidly declining monarch butterfly (Danaus plexippus) and other beautiful pollinators is more widely recognized, but it is important to note that the stems and seeds of the plant have ecological value beyond its prime bloom time. Common milkweed flowers are boisterously showy and fragrant, but keeping the sturdy stems, fading leaves and seed pods standing after blooms’ end is an important ecological practice and requires us to expand on what we consider beautiful to encompass more aspects of our ecosystems and all of the wondrous life cycles contained within. For small-space gardeners, butterfly weed (Asclepias tuberosa) is a somewhat more demure but also beneficial and beautiful native milkweed to consider planting, but research has shown that monarchs lay the most eggs on common and swamp milkweed (Asclepias incarnata).

Allowing plants to reproduce by seed maximizes genetic diversity and helps avoid having monocultures in your landscape, an important sustainable landscaping principle. Genetic diversity is required to allow plant populations to adapt to variable and changing climate conditions. Too much of the same genetics can result from strictly practicing vegetative propagation from cuttings or plant divisions, leaving plants vulnerable to things like disease, pests, reproductive failure, or climate extremes of drought or flood. So let those fall milkweeds stand and go to seed. There’s a good chance that the stems will also become host to beneficial pollinators like native bees over the winter, as well.

Why divide perennials, then, you say?

Well, divisions are necessary to keep some perennial plant species healthy, to stimulate new growth, reduce crowding, and sustainably manage garden beds. Robin Wall Kimmerer, author of Braiding Sweetgrass: Indigenous Wisdom, Scientific Knowledge, and the Teachings of Plants writes, “Our elders taught that the relationship between plants and humans must be one of balance. People can take too much and exceed the capacity of the plants to share again. That’s the voice or hard experience that resonates in the teachings of “never take more than half.” And yet, they also teach that we can take too little. If we allow traditions to die, relationships to fade, the land will suffer. These laws are the product of hard experience, of past mistakes. And not all plants are the same; each has its own way of regenerating.”

Carefully dividing perennials can be a sustainable and economical way to expand your native plant coverage in your landscape and keep your plants healthy. Native plant exchanges across neighborhoods are one strategy to boost genetic diversity if you’re interested in spreading the native plant love beyond your own yard by safely sharing your native plants. We look forward to establishing a Bay-Wise Native Plant Exchange via social media in the near future, so watch this space for more details to come.

When should I divide perennials?

In general, you’ll want to avoid dividing plants while blooming. The cooler temperatures of spring and fall also help reduce heat stress on the plant. It will be easier to dig and separate out plants at the roots following a rain. I know what I’ll be doing this weekend! I have a sprawling patch of woodland sunflowers (Helianthus divaricatus) along my driveway that were a veritable AirB&Bee all summer long that have now gone to seed, so it’s time to thin them out and expand my sunflower bed to absorb more of our stormwater runoff.

If native grasses are your thing, cool-season grasses are best divided in the early fall, warm-season grasses in early spring. More details about when and how to divide different types of perennials can be found on Clemson University’s website.

The great divide

To get started, you will first need to decide if you are going to pot or immediately relocate your divides. If you are potting up your divides, set up a simple potting station nearby with small pots, a transplant tool or knife, loose, clean soil, and compost or leaf mulch. I like Leaf Gro because it is a Maryland sustainable product made from locally collected leaves. You may want to have a water mister bottle or some wet cloth (burlap or a light cotton towel or damp sheet will do) or paper to protect the exposed roots, as well. If you are relocating the divides elsewhere in your landscaping, prepare your planting hole ahead of time by digging double the volume as the root ball and sprinkling in layers of leaf mulch and loose soil. 

Now you are ready to divide the plant. If the soil is damp, this whole process will be easier on you and the plant. Gently dig up an outer portion of your native plant, working your way around the entire section with the shovel, assessing root depth as you go. Try to get all the way under the root tips before lifting the plant out of the ground. Think vertically rather than horizontally. Our aim is to divide the root system but not to cut the roots short.

When dividing or transplanting any native plants to transplant within your yard, you may want to keep some of the soil that the plant is currently rooted in, as the roots tend to establish important relationships with local soil microbes and fungi. If you wish to share with your neighbors and Bay-Wise Community, though, please be mindful of not spreading weed seeds or pests like invasive jumping worms! Don’t share soil from your yard if you know jumping worms or other invasives to be present. If you are unsure about the presence of invasives in your yard, bare root plants in sterile potting soil instead, to be safe. Mindfully sharing divided perennials is just one way of spreading the native plant love for a climate-resilient and healthy Chesapeake Bay landscape! 

After digging up a small portion of the plant, gently knock off chunks of soil back into the hole you dug it from with your planting tool or fingers. Shake loose soil right back into the hole with some leaf mulch to repair the garden bed and gently work excess soil out of the roots with your fingers.  

Keep the roots moist as you work, using your damp cloth, paper, or mister bottle, and have your new planting hole prepared for quick relocation. If immediate relocation isn’t your goal, have your planting pots and tray, clean water bucket, sterile potting soil, and compost ready at your planting station.

You’re now ready to divide the root ball with a transplanting tool or knife into smaller sections, being careful to wear good gardening gloves, or you can gently work the roots apart with your fingers if you’re working with small clumps. At this point, you can remove any dead vegetation and trim the green portions back to about 6 inches. Re-pot in clean, small pots with loose, sterile soil and a sprinkle of leaf mulch. 

Now you have native plants to safely share or relocate to expand your pollinator or rain garden!

QUICK TIPS: Try to get photos of your plant in bloom to share in your plant exchange or for your own garden design needs. Label the pots or mark your garden map so that you don’t forget what you have over the winter!

By Stacy Small-Lorenz, Agent, Residential Landscape Ecology, University of Maryland Extension. Read more posts by Stacy.