How and When to Divide Perennials

How and When to Divide Perennials

Apr 12, 2019

Perennial plants are true treasures of the garden. For many flower gardeners, perennials are anchor pieces. They put on tremendous shows of bloom and foliage, somehow doing it year after year. That’s because perennials have nature’s wonderful gift of living long and being prosperous.

Perennials are one of the best plant investments any gardener can make. Unlike annuals that need yearly replacement, perennials volunteer every year. They show up in the spring, bloom through the summer and then retire come fall. Perennials seem to die off in the winter, but that’s just a disguise.

Under the cold snow and ice, perennial plants lie dormant. They conserve energy and store it for when soils warm and days lengthen. Then, as if by magic, perennials pop themselves above the ground and begin vigorous displays of leaves and flowers. That’s a tremendous return on an initial investment. It’s one that pays back for years on end.

There’s just one problem with perennial plants. They can be too vigorous. Healthy perennials have a nasty habit of taking over garden space. They encroach on other plants, crowding out competitors. Perennials can even be their own worst enemy as massive clumps of roots and stalks starve themselves for space and energy.

Out of control perennials begin to stress. They show decay in their centers and edges. Left unchecked, perennials can fade from prosperity. They’ve done their duty and prepare to move on in nature’s lifecycle. But there’s an easy way to prevent perennials from aging — you simply divide them.

Why Divide Perennials?

Experienced gardeners know how important it is to divide their perennial plants. Many green thumbs learned when to divide perennials, or how to separate perennials from other gardeners. Some are self-taught and picked up tricks of the trade by trial and error. No matter where they learned about splitting perennials, all good gardeners should know the reasons why perennial plants need periodic dividing:

  • Controlling Plant Size: Most perennial plants continue to grow and expand if unchecked. Some perennials are so energetic that they double or triple their size yearly. That might look great for show, but the plant is dooming itself with expansion. Dividing perennials keeps their size under control and maintains good plant health.
  • Rejuvenating Perennials: Older, denser perennial plants become root bound and restrained. They react by dwindling in size, distressing at their centers and producing less show. Once divided and freed-up, perennials show remarkable rejuvenation. It’s equivalent to them being reborn.
  • Multiplication and Reproduction: New plant groups quickly form and flourish into additional clumps with emerald foliage and rainbows of flowers. These clone-like offspring carry on spreading their parents’ genes and lineage. This becomes an endless cycle if you properly divide perennials.
  • Setting Boundaries: You can create boundaries by separating and spacing perennials within garden limits. Most avid gardeners take pride in strategically setting their perennial plants along outer boundaries. Their permanent presence marks specific territories that define areas for show and practicality. Dividing perennials lets gardeners control the spread and spacing that takes full advantage of additional plants.
  • Establishing Beauty: You can add beauty by digging up perennials, cutting them apart and moving them to new locations. That might sound like harsh plant abuse, but it’s a beautiful thing for perennials. Freshly divided perennials put on new and beautiful faces that tired, old plants can’t provide. There’s nothing like youth to facelift the garden.
  • Making Friends: If there’s a gift that keeps on giving, it’s perennial plants. Their prolific rate of reproduction ensures perennial plants expand their family and offer new life in other gardens. Many cooperative neighbors and friends routinely divide their perennials and share them with each other. It’s one of the most neighborly things you can do.
  • Adhering to Nature: Division is harvesting nature’s garden bounty. It’s nature’s plant bonus system and wouldn’t be designed that way if not natural.

What Are Perennial Plants?

Before going into the garden and digging away at perennial plants, it’s helpful to know exactly what they are. Knowing when to split perennials and how to divide perennials is important, but it’s also nice to understand what these fountains of youth are and how they operate. Here’s a bit of perennial information.

Perennial means “per-annual” or “through the years.” Perennials are a plant classification that identifies species that live more than two years. They keep living continuously for many years without having to reproduce themselves through seeds or propagation. Annuals are a specific plant group needing yearly replacement. They also have two-year cousins called biannual plants that have a semi-perennial nature.

True perennials have an almost indefinite lifespan. If regularly divided, perennials are known to outlast human lives and follow generation after generation in providing garden service. That’s provided the right care and attention is given, particularly in separating them, which is their true key to long life.

Botanically, perennials are polycarpic or iteroparous plants that flower yearly. They continue life through a chemical process that forms internal vegetable reproduction rather than condensing its genetic imprint in seed pods and passing it down. Perennial continuation happens in root systems rather than seed pods. Root types vary from bulbs, tuber, woody crowns, rhizomes and others.

Types of Perennial Plants

There are two main perennial plant types. They’re seen every day around the world, although most people don’t stop to appreciate the difference. But avid gardeners do. They know the two perennial plant types are grouped as:

Evergreen Perennials: These are everywhere. Trees are the perfect evergreen example even though some are deciduous trees that seasonally loose leaves. Gardeners are more familiar with smaller shrubs as perennials. These are above ground plants go dormant over winter and reappear full-force in the spring.

Herbaceous Plants: These are what most people think of as perennials. Herbaceous perennials are the ones that appear to die off above ground and leave a mess of decaying vegetation to show for their summer work. That’s just a process for herbaceous perennials. They’re very much alive below the surface and getting ready to show once it’s spring.

Perennial plants have huge variations in size, tolerance and hardiness. That includes both evergreen and herbaceous perennials. One of the biggest factors for perennial prosperity is making sure the plant is suitable for its climatic zone.

The United States Department of Agriculture (USDA) has identified eleven different climate zones across the nation. That includes Hawaii and Alaska. Zone ratings go from 1, where it’s too cold to grow practically any perennial, to 11, which has the warmest year-round temperatures. It’s vital that perennials are matched to their climatic zone. These zones are easily identified by entering a location zip code into the USDA climate zone site.

Perennial plants are also broken into specific categories. Certain perennials are sun lovers. Others are shade dwellers. Some survive harsh winter conditions while others are drought and deer resistant. Perennials also have different bloom times. That ranges from spring to fall and has an important effect when it comes to dividing time.

Indicators for When to Divide Perennials

One great thing about perennial plants is they’ll give plenty of indication of when they need dividing. Experienced gardeners recognize the telltale signs of an aging perennial and know when it’s time to split. For most perennials, this will happen every three to five years. Some like dividing every two to three years. Some species can go for a decade without being broken up. And there are a few ornery perennials that simply want to be left alone.

It’s best to confer with a good perennial guide to identify exactly what perennial species is best suited to yearly division. It’s also helpful to get advice from a reputable garden center with experts who know perennials inside out. But there’s nothing like listening to the perennials themselves and watching what state they’re displaying. Here are some indications showing when it’s time to divide perennials:

  • Vigorous, Uncontrolled Growth: This is the main sign that a perennial is asking to be separated. Healthy perennial plants can expand at an enormous rate and quickly outgrow their designated area. Once perennials become too crowded, they’ll begin to stress, which leads to declining health.
  • Neighboring Plants Resisting Expansion: This is another clear indicator that a perennial needs to be put in check. This can mean adjacent plants being overcome or simply receding into their own confines. These next-door neighbors will also show shock signs and tattletale that the infringing perennial needs dividing.
  • Shock Signs: Two shock sign include reduced leaf and flower production. When perennials become stressed by overcrowding, they’ll react by shrinking their foliage and blooms. This is a natural reaction when insufficient space prevents them from flourishing. Fortunately, shock is easily reversed by digging up perennials and splitting them into multiple sub-plants.
  • Depressed and Dying Centers: It’s clear when the plant’s center section is failing. Outer edges will still seem healthy, but at its heart, the plant is weak and needs to be divided.
  • Weak or Missing Bottom Foliage: This shows a perennial being stressed. Competition for energy and space due to overcrowding results in lower leaves being sparse and light colored. That’s a red flag that the perennial plant needs to come up, be split and replanted into wider spaces where it can continue its endless expansion.

Seasons for Dividing Perennials

Here’s an area where practically all perennial gardeners agree — both old pros and newbies alike. All advice, no matter where it comes from, points to a universal or perennial rule. It’s not a rule-of-green-thumb. It’s a botanical fact.

Perennial plants that bloom in the spring should be divided in the fall. Conversely, fall-blooming perennials have to be separated in spring. That’s because of energy conservation.

When a perennial plant is putting its energy into blossoming, it can’t easily take the stress of being uprooted, torn apart and transplanted.

Dividing perennials at the wrong time of year puts needless stress on the poor plant. It’s sure to go into shock, and the flowers will suffer. The plant will naturally put its energy into establishing a new root system to ensure long-term survival.

This year’s blooms will be shed, and there is good chance flowers for the next year or two will suffer as well.

It’s not enough to say divide in spring and fall. Each season’s timing needs individual dividing, and it can get specific to the individual plant species. Generally, spring dividing should be done before much of the above-ground growth has appeared. Certainly, before plants form flowers buds.

Fall dividing has the best success when flowers are wilting but before leaves start to droop.

Your climatic zone has a big effect on dividing time. Whether spring or fall, the best guideline for separating perennials is when the soil is warmer than air for at least part of a 24 hour period. Some say it’s just before daffodils are at peak season, or right when fall nights start to leave morning dew. On both ends, a gardener can’t go wrong when conditions are 50 percent humidity and 50 degrees Fahrenheit.

There are exceptions to dividing time, of course. Some perennial plants can be dug up and divided in full bloom just as some perennials can’t be divided at all. This solely depends on the species. No matter what perennial, it’s always wise to check credible information before upsetting the plant. But one thing no perennial can withstand is going without water in their root systems when being divided.

Perennial Plant Root Systems

The key to understanding perennial plants lies in their unique root systems. Perennial roots don’t just hold the plant in the ground. Roots hold the perennial’s genetic coding or its DNA sequence. This living information lets the plant respond as nature designed it. That means converting water and soil nutrients to develop stalks and leaves that act with sunlight to photosynthesize energy, converting it to flowers and foliage — year after year.

Perennials have five main types of root systems. Each type has its idiosyncrasies when it comes to dividing. Fortunately, most perennials are hardy plants, and as long as basic rules are followed, they’re easy to divide and have a high survival rate. Here are the five main perennial root systems:

  1. Surface Roots: These run parallel to the ground surface. Some are slightly above the soil. Others are subsurface but only a few inches deep. Surface root perennials propagate by sending runners out to find open space. Here, they pop up and form new plant clusters. These plants are separated by cutting the root connection between the parent and child.
  2. Underground Running Roots: Often called suckers, underground running roots are subterranean dwellers that grow under their mother plant’s shadow. Underground runners are a bit more difficult to divide as they require excavating the entire parent plant and pulling away the suckers.
  3. Clump and Offset Roots: These are actually new clusters of perennial plants. In new generations, clumps are close to their parent but can easily be cut off, dug out and transplanted to a new location.
  4. Woody Roots: Woody roots are more common with evergreen perennials, but herbaceous plants can have woody roots, too. They’re notable for being large, tough mangles of root balls that extend from the surface to well down below. Woody root plants are divided by cutting the stem between the old and new growth.
  5. Tap Roots: Tap roots are the most difficult ones to divide. These perennials are usually the drought-resistant species that send one single spike root deep down to the water table. The division depends on extracting the main plant and severing the tap root down the middle.

The successful perennial division is all about root management. If you properly sever roots and carefully handle them during separation and relocation, the chances of plant survival are excellent. But this comes with a qualifier. There’s a five-step process to follow for successfully dividing perennials.

The 5-Step Perennial Dividing Process

Like every process, dividing perennial plants works best if steps are followed in order. This simply makes a process easier to understand and repeat. There’s nothing to be hesitant about in dividing perennials. The process is simple and virtually guaranteed:

  1. Prepare: The more time spent preparing to divide perennials, the easier the other steps will be. First is planning the time and date. It doesn’t matter if perennials are spring or fall candidates, they’re both best divided on cool, cloudy days. The midday heat is a killer on disturbed roots. Make sure the mother plant is well watered before excavation. Also make sure all tools, containers and transplanting aids are nearby. The less time roots are exposed, the quicker they’ll take hold and rejuvenate.
  2. Lift: Pay attention to the parent plant’s drip line. That’s the outer edge of its foliage canopy. This is where the sensitive roots extend out to. First, dig around the circumference of the drip line and then begin to undermine the plant. Small plants can be lifted by a hand trowel, but big, established perennials will need a spade or garden fork. Remove the whole plant and place it in a tray or on a tarp to catch native soil.
  3. Separate: This is actually dividing the mother plant into sections. Depending on root type, separating can be done by hand tearing or simply pulling the root ball apart. Others may need mechanical separation with shears or a knife. Don’t be afraid to shake off excessive soil as long as it’s collected. Light hosing works well, too. This gives a clear opportunity to inspect for disease or insect issues.
  4. Plant: Proper planning ensures the divided perennial’s new location is already prepared. Moist organic soil should be in a place that contains suitable fertilizer and rooting hormones. A new bed should be at least twice the diameter of the divided perennial. This gives unobstructed room for roots to quickly expand and take hold. Divisions should always be planted on a slight mound above the general ground level. This lets roots naturally grown downward.
  5. Maintain: Newly divided and transplanted perennials are sensitive for a few weeks until they get settled. During this time, water is critical. You should never allow perennial roots to dry during division, especially when they’re taking new hold, but also try not to overwater as this lets the roots get lazy. Experts always recommend mulching but use clean mulch that’s free of pests or disease. New transplants won’t have the same resistance as established perennials. The temporary shade is also a good idea when there’s a chance of temperature spikes.

These five dividing steps have the best outcome when the gardener starts with quality perennial plants. There’s nothing like a healthy start to every garden, whether it’s acquiring new plants from a reputable grower or dividing perennials originating from a greenhouse supply company. That’s the best return on investment any green-thumber can make. It’s rewarded by perennial blooms that last generations.

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