Although their name may suggest otherwise, perennial beds and borders do change with time. Every few years they need to be reevaluated. Plants may need to be moved to a different location where they will either look or grow better. A great number of perennials (at least the ones I grow) benefit from division. Perhaps those trees nearby have extended their branches enough to alter the amount of sunlight now available. Or maybe you lost part or all of a shade tree in last year’s wind storms. Maybe that cute little 4-inch pot of doronicum you planted has laid claim to more than its fair share of the garden. Even your tastes in colors, design ideas or seasons of bloom may have changed. Whatever the reason, spring is a good time to overhaul the perennial garden.

Elm broken by last year’s wind storm. Photo by dmp2020

Before you begin to pick up that spade and begin digging, you need to decide on the type of look you are aiming for. Your site conditions will likely dictate your choice of plant material. Try as you might, perennials like gaillardia, lavender and dianthus will not do well in soggy soils, while hellebores will wither away in hot, dry exposed sites. Consult one of the many splendid books or websites on perennials, talk to a knowledgeable person at a local garden center, or give us a call if you are in doubt about a plant’s cultural requirements.

Another factor to consider is the maintenance many perennials require for their best display. Delphiniums in all but the most sheltered areas need to be staked. Yarrows and evening primroses should be divided every couple of years. Garden phlox must be religiously deadheaded so its usually magenta colored progeny do not take over the world. Lilies need the once over just about every day to patrol for lily leaf beetles. And, some plants like columbine, rudbeckia and agastache just seem to have relatively short life spans, at least in my yard, and require regular replacement whether through self-seeding or store purchase. I don’t believe a plant exists that does not require at least occasional attention but if you are limited in the amount of time you have to deadhead, stake, divide, and control pests, you will definitely want to choose less demanding perennial species.

Birdhouse garden with desired coral colored phlox and self-seeded magenta phlox. Photo by dmp2009

When redoing your perennial beds, keep in mind also the season of bloom. Many perennials, for all their loveliness, have a tendency to bloom over a short 3 to 6-week span of time. A few will provide color, or at least interest from early summer until frost. These include plants like hostas, coral bells, Russian sage, and some dwarf daylily cultivars. Some gardeners strive for a riot of color for mainly one time period, say the month of June, while others prefer smaller portions of color that extend over the whole growing season. Spring flowering bulbs and annuals can provide interest either by complementing the flowers of perennials or as fillers when little else is blooming. 

Sedum, coral bells and artemesia provide a long season of color. Photo by dmp2012

Spring is generally a great time to divide mid and late season flowering perennials with the early fall being better suited for the early spring bloomers. If you cannot replant the divisions immediately, pot them up, or heel them in somewhere not in full sun. Perennials with a long tap root like baby’s breath and echinops do not appreciate being moved so place them carefully. Asters and some others in the composite family tend to die out in the center. Just transplant the new growth surrounding it and discard the woody middle part. If you need to move Oriental poppies, wait until they go dormant, usually in July or August.

Asters multiply quickly and benefit from division every 3 years or so. Photo by dmp2012.

Think about what bulbs you might like to see blooming with your early season perennials and make a note to purchase them for fall planting. Place the bulbs behind sprawling perennials so that the dying bulb foliage will be camouflaged.

Daffodils and other bulbs in the birdhouse garden and white garden. Photo by dmp2013.

Don’t be afraid to experiment and, most of all, don’t be afraid to rectify any unsatisfactory plantings. Unlike, permanent tree and shrub plantings, a perennial garden can be modified to suit your needs and desires.

Happy Spring!

Dawn P.

Many years ago I received a flat of Iris rhizomes from a friend when she was dividing clumps that had outgrown the area in which they were planted. I planted them in the flower border that runs across the back of our yard. For many years they have bloomed, each year putting on a larger show. The past two or three years I have thought that it may be time to divide and replant them. Unfortunately that thought happens when the iris is in full bloom. When the appropriate time comes to divide and replant I have usually moved on to the many chores of summer not the least of which are the high demands of the vegetable garden. But not this year. Iris can be divided any time from late July through September so I ask: is September too late to keep a resolution? I don’t think so. This year I divide (and conquer) the iris.

A crowded planting

There are more than 200 species of iris but the most common in our area are the bearded and Siberian iris. Iris are easy to grow hardy perennials but if they become crowded then they are more disease and insect-prone and flower production is reduced.

I already know that the flower bed in our yard is a good spot for the iris as it is receives full sun and is well-drained. Selecting new areas to put the divided rhizomes into is not a problem. It is always best to have the new site ready to go so that the rhizomes aren’t drying out in the sun while a new plot is dug. The new hole should be about 5” deep with a small mound in the middle of the hole (you will see why in a bit).

The new hole, 5

The new hole, 5″ deep with a center mound

Also, have ready a few marked trays or buckets to put the rhizomes into if you would like to keep track of the different varieties/colors. I have plenty of pictures of the Iris in bloom so that I have a good idea of where the colors currently are in the bed even though all that I can see right now are the leaves.

Tools for the job

Tools for the job

So, what are tools needed for this job? A spade or a digging fork, a pair of shears, a sharp knife, and a bucket of 1:10 bleach/water solution. The first step in the actual dividing process is to cut the leaves back to a third of their height using a pair of garden shears, trimming them into a fan shape. Be sure to dip the shears into the bleach solution often to avoid spreading diseases.  

Cutting the leaves

Cutting the leaves

1/3 of the original height

1/3 of the original height

Next, lift out the entire clump that is going to be divided by getting underneath it with the spade or digging fork. You may need to work around it in a few areas to get under it.

Removing the clump

Removing the clump

Place the entire clump on the ground and take a look at it to decide where it should be divided. It is easy to see which rhizomes have new growth  and which are no longer supporting  any foliage. Using the sharp knife cut away each section of rhizome that will be replanted, dipping the knife into the bleach solution often to disinfect it. Each new rhizome section should have plenty of roots and a fan of leaves.

Cutting the new rhizome

Cutting the new rhizome

At this time it is important to check for iris borers and  the bacterial soft rot that often accompanies them. Each rhizome should feel firm when pressure is applied. If the rhizome gives way easily then that is a sign that there may be a borer. The adult iris borer is a brown and grey moth. It lays its eggs on the iris leaves and plant debris at the base of the iris in the fall where they will overwinter and hatch into tiny caterpillars in April and May. The new caterpillars will crawl up the foliage, chew pinprick sized holes and begin to tunnel their way back down toward the rhizome. Signs of their feeding are streaks that appear tan or water-soaked. By mid-summer the borers can be up to 2” in length and have reached the rhizome. Their feeding allows the entry of the bacterial soft rot that turns the rhizome into a smelly, mushy, mess. Late summer will see the borers moving into the soil to pupate, emerging as adult moths in the early fall. There are some varieties of the Siberian iris that are more tolerant to a borer attack.  

Possible borer activity

Possible borer activity

Scouting and sanitation practices can be the most useful controls.  Look for the tell-tale signs of chewing damage and water-soaked streaks in the spring when it is easy to crush the insect while it is still in the leaf or remove the leaf entirely. If the plant has above ground symptoms in July then dig up and examine the rhizome for signs of borer activity, discarding any that are infested. In the fall remove all plant debris where the eggs might overwinter. If you have an infestation that you feel is severe enough to warrant an insecticide then acephate (highly toxic to bees when freshly sprayed and as a residue) and spinosad (non-toxic to bees when it is dry) are generally recommended. Here is a link to the Virginia Cooperative Extension fact sheet which also includes some images: Iris Borer

And now back to the rhizomes. It’s time to place them into the prepared holes, putting one rhizome section on top of the mound in each hole. Spread the roots out and down and fill the hole with soil. Be sure that you don’t bury the rhizome; it should still be visible from the surface.

Iris rhizome in its new hole

Iris rhizome in its new hole

The fans of leaves are usually planted so that they all face in the same direction. Water each plant thoroughly and in years such as this one, more than once. Newly transplanted iris may need a winter cover of straw to keep the newly planted rhizome from coming out of its new location due to  the thawing/freezing cycles that can happen. Just be sure to remove the straw in the spring. These new plants may not bloom much the first year but after that they should be back to their normal, showy selves.

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Susan Pelton