Divide These 8 Plants in August – It's the Secret to Bigger, Healthier Borders Next Year

August is an excellent time to divide certain tired-looking perennials, including bearded iris, daylilies, hostas, Shasta daisies, garden phlox, Oriental poppies, hardy geraniums, and astilbe. Choose a mild, overcast day, water well afterward, and make sure every new piece has healthy roots and shoots so it can settle before frost.

The telltale signs are a bare center, fewer flowers, flopping stems, or a clump that has simply become too large for its allotted patch of earth. Plants, like dinner-party guests, are much more charming when they are not sitting on top of one another.

August can be busy with sowing and planting, too, and there is plenty of inspiration in what to plant in August while you refresh the borders. For the technique itself, use the same principles explained in how to divide plants: lift, split, replant promptly, and water deeply. A sturdy border spade from Lowe’s and a hand fork from Amazon will make the job much easier.

How to Tell If a Perennial Is Ready to Divide

I tend to approach division with a mixture of optimism and a faint feeling that I am about to perform garden surgery without a medical degree. Fortunately, most established perennials are obliging patients. If a clump has a bald middle, produces smaller flowers than it once did, or has spread beyond its welcome, it is usually asking to be lifted and shared.

Late summer is most suitable for plants that have finished flowering and can make roots in warm soil before cold weather arrives. In very hot, dry regions, particularly USDA Zones 8 and above, postpone the work until temperatures soften or wait for early fall.

Bearded Iris: Bring Their Rhizomes Back to Life

Bearded iris (Iris germanica) are among the classic plants to divide in August. Their thick, knobbly rhizomes creep along the soil surface with all the discretion of a small, determined lizard, and crowded colonies soon lose their flowering swagger.

Lift the rhizomes with a fork, shake away excess soil, and cut them into pieces with a fan of leaves and a firm section of healthy rhizome attached. Trim the leaves back to around 6 inches, which helps reduce water loss while the roots re-establish. Discard any soft, hollow, or suspiciously squishy pieces; sentimentality has no place in rhizome management.

Replant each division shallowly, with the upper surface of the rhizome visible or only lightly covered in well-drained soil. This is also a sensible moment to revisit what irises need and make sure a once-floriferous clump has not been buried too deeply or shaded by enthusiastic neighbors.

Daylilies: They Reward a Little Tough Love

Daylilies (Hemerocallis) have a reputation for being nearly indestructible, which is reassuring because they can require a fair amount of muscle when mature. August division works well after flowering, especially in temperate USDA Zones 4–8, giving new plants time to root before winter.

A clump ready to divide often has congested crowns and fewer flowers around its center. Lift it, hose or shake off enough soil to see the roots, then tease sections apart by hand. If that proves impossible, use two garden forks back-to-back or cut between the crowns with a sharp spade. Each division needs a healthy fan of leaves and a generous root system.

Plant divisions at the same depth as before and keep the ground evenly moist for several weeks. Daylilies make particularly satisfying “free plants” for filling gaps, edging a path, or passing to a neighbor, provided that neighbor has not already started crossing the street to avoid your surplus zucchini. Their easy-going nature earns them a place among the most useful perennial flowers that will last for a long-season display.

Hostas: Divide Before They Become a Green Traffic Jam

Hostas can be divided in late summer in cooler or reliably watered gardens, though I avoid doing it during a heatwave. Their broad leaves lose moisture quickly, so choose a cloudy day and work efficiently. In warm climates, early fall is often kinder.

A hosta that has formed a vast, crowded mound may still look handsome, but division can improve airflow and give you several substantial new plants. Lift the root ball, then cut down through the crown with a clean spade or an old serrated knife.

There is no need to reduce a majestic specimen to dozens of sad little fragments; three or four generous divisions will establish faster and look far more convincing.

Water the replanted sections deeply and mulch with compost, keeping it clear of the crown. A bag of organic compost from Plant Addicts is useful if your soil needs improving, particularly beneath thirsty hostas. Their luscious foliage is one reason shade friendly plants need never feel like an apology for a sunny border elsewhere.

Shasta Daisies: Make More Room for Sunshine

Shasta daisies (Leucanthemum × superbum) are bright, cheerful, and after a few years, capable of becoming a rather impenetrable white-and-green scrum. Divide them after the main flowering flush, removing exhausted stems first so you can see what is going on.

Lift the clump and look for younger, vigorous growth around the outer edge. These are the pieces worth keeping. The older, woody center can be composted if it has lost its enthusiasm. Replant healthy divisions about 12–18 inches apart in full sun, watering them carefully while roots knit into the soil.

This is a perfect opportunity to move a few divisions into a sparse section of border. August is not the month for a great deal of botanical overthinking: a well-placed daisy can be surprisingly transformative, especially alongside the late summer flowers that keep a back yard looking lively as fall approaches.

Garden Phlox: It Deserves a Breath of Air

Garden phlox (Phlox paniculata) brings wonderful color and fragrance to late-summer borders, but congested clumps can become more vulnerable to powdery mildew. Division is not a cure-all - good spacing and sun matter too - but it is a useful reset for plants that have been standing shoulder to shoulder for too long.

Once flowering is fading, lift the outer sections of the clump and divide them into pieces with roots and several shoots. Replant in fertile, moisture-retentive soil, allowing plenty of room between plants. This is not the moment to create a phlox convention. Airflow is vital.

I always label new divisions immediately, because every gardener is convinced they will remember exactly where the pale pink phlox went until next May, when the border becomes a botanical witness-protection program. These weatherproof plant labels from Amazon are a small but sanity-preserving investment.

Oriental Poppies: They Prefer a Quiet August Move

Oriental poppies (Papaver orientale) disappear after flowering in a dramatic annual vanishing act, leaving an empty patch of soil just when you have become fond of them. Their late-summer dormancy makes August a good time to lift and divide mature clumps, provided the soil is not parched.

Look for fleshy root sections with buds or small shoots. Cut these into substantial pieces and replant promptly, setting them in a sunny position with well-drained soil. They may look unimpressive for a while, but patience is part of the deal with Oriental poppies; they reserve their grand entrance for spring and early summer.

Do not overwater divisions in heavy soil, as the roots dislike sitting wet. A hori hori garden knife from Amazon is particularly useful here, slipping neatly between roots and doubling as a remarkably satisfying weeding tool. Their theatrical blooms are well worth considering when planning a more generous flower bed next year.

Hardy Geraniums: They Can Be Rejuvenated and Shared

Hardy geraniums are among the most useful, unfussy plants in a temperate back yard, tumbling over edges and politely knitting borders together. They can also become woody and tired in the middle, which is their discreet way of asking for attention.

After flowering, cut back the foliage, lift the clump, and divide it into healthy rooted sections. Replant in moisture-retentive but well-drained soil, then water consistently until new growth appears. If the plant is still flowering strongly in August, leave it alone and divide it in early fall instead.

The great pleasure of dividing hardy geraniums is that the results immediately look generous. One faded clump can become several fresh drifts, useful for weaving through roses, shrubs, or other plants that need their ankles elegantly covered.

They are particularly valuable in relaxed cottage garden planting schemes, where a little soft sprawl is entirely welcome.

Astilbe: It Benefits from a Fresh Start in Moist Soil

Astilbe is a fine candidate for division in late August or early fall in cool, moist conditions. It dislikes drought, so this is one plant I would not tackle after weeks without rain unless you are prepared to water diligently.

A mature clump may develop a woody center and weaker flowering. Lift it carefully, split it into robust sections with buds and roots, and replant promptly in humus-rich soil. Astilbe is happiest in partial shade, where its ferny foliage and feathery flowers lend a border a wonderfully composed air.

Mulch around new divisions with leaf mold or compost, keeping the crowns clear. A garden trug from Amazon makes carrying divisions, compost, and the inevitable collection of abandoned plant labels rather easier. In damp shade, astilbe partners beautifully with hostas and ferns; proof that shade friendly plants can be every bit as glamorous as sun lovers.

The real secret to dividing plants in August is brisk kindness. Do not leave roots baking in the sun while you admire your progress, and do not expect small divisions to fend for themselves without water. Replant at their previous depth, firm the soil gently, water thoroughly, and mulch if conditions are dry.

Keep new plants watered through the first few weeks, especially if rain fails to appear. Remove flowers if a divided perennial immediately tries to bloom; it is far better for it to spend its energy making roots. By fall, those once-crowded clumps will be settled, stronger, and multiplied into useful new stock.

It is one of gardening’s finest small miracles: a plant that looked past its best becomes three or four plants full of promise. No expensive makeover, no dramatic intervention, just a fork, a little compost, and the agreeable knowledge that next year’s border will be better because you gave it room to breathe.

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