When to Trim Bushes in Illinois: A Shrub-by-Shrub Calendar

When to trim bushes in Illinois depends on when each shrub flowers. Spring bloomers like lilac, forsythia and weigela get trimmed right after their flowers fade. Summer bloomers like smooth and panicle hydrangea, potentilla and Japanese spirea get trimmed in late winter or early spring, and evergreens like yew, juniper and arborvitae get shaped in March or April before new growth starts. The window to avoid for real pruning is late summer through fall, when fresh growth cannot harden off before winter.

The Rule That Matters: When Does It Bloom?

Every shrub in your yard falls into one of three groups, and the group decides the timing.

• Spring bloomers flower on old wood. Lilac, forsythia, spring spirea, weigela, ninebark and viburnum set next year's flower buds the season before. Trim them soon after the flowers fade. University of Illinois Extension notes most spring bloomers are done flowering by the end of June.
• Summer bloomers flower on new wood. Illinois Extension defines them as shrubs that bloom after July 1. Think potentilla, Japanese spirea, rose of sharon, smooth and panicle hydrangea. They grow this year's flowers on this year's stems, so trim them in late winter or early spring.
• Evergreens and foliage shrubs. Yew, boxwood, arborvitae and juniper are shaped before spring growth starts. Burning bush and barberry, grown for leaves and color rather than flowers, are pruned while dormant.

Dead, broken or diseased branches are the exception to every rule. Cut them out whenever you see them.

Why Timing Matters in Will County

The Illinois State Climatologist notes the 2023 USDA hardiness map expanded zone 6a across the Chicago region, from Lake County down to Kankakee County. Milder than it used to be, but every fall still ends in hard freezes. Illinois Extension warns that pruning deciduous shrubs in late summer causes a flush of new growth that cannot harden off before winter, which leaves it open to frost damage and dieback. For evergreens it is blunter: fall pruning is not recommended.

A typical foundation planting in a Manhattan, New Lenox, Frankfort or Mokena subdivision is a mix: yews or boxwood by the door, a burning bush or spirea at the corner, arborvitae on the lot line, a hydrangea or lilac somewhere. That mix is why one trim date per year never suits every shrub in the bed.

Best Time to Trim Bushes, Shrub by Shrub

Trimming Evergreen Bushes: Yews, Boxwood, Arborvitae, Junipers

The Morton Arboretum's rule for most evergreens is to prune just before spring growth begins and to avoid late summer and fall. Shearing makes the outer foliage denser, which shades the inside and grows a "dead zone" of bare wood that will not resprout. Each evergreen handles that differently.

When to prune yews

Late winter or early spring, then an optional light trim in mid-June, and nothing after July. Yews sprout from old wood, so an overgrown yew can be cut back hard. Penn State Extension notes that yew foliage and berries are poisonous to people and pets, so clean up every clipping.

When to trim boxwoods

Chicago Botanic Garden says March is fine for healthy boxwood. If winter left brown foliage, wait for new growth, then cut out the browned branches. Light shaping can continue into early summer.

Emerald Green arborvitae pruning

Can arborvitae be pruned? Yes, and Morton notes it responds well because new shoots grow from hidden buds in the branch crotches. The catch is the brown interior: Penn State Extension says that among these evergreens only yews have dormant buds on old wood. Keep cuts in the green. Morton lists Emerald Green at 10 to 15 feet tall and 3 to 4 feet wide, so one planted under a window is a lifelong fight. If it has outgrown its spot by several feet, removing and replacing it usually beats hacking it into bare wood.

Junipers

Prune in March or April. Morton recommends against severe pruning and suggests cutting at the V-shaped forks where branches split. Tip-pruning keeps the feathery look that shearing destroys.

Cutting Back Hydrangeas (and Why Bigleaf Is Different)

Trimming hydrangeas starts with knowing which kind you have.

• Smooth hydrangea (round white heads, the classic Annabelle type) blooms on new wood. Iowa State Extension says it can be cut to the ground in March or early April.
• Panicle hydrangea (cone-shaped flower heads) also blooms on new wood. Illinois Extension says you can remove a third to half of the shrub in late winter or early spring.
• Bigleaf hydrangea (the blue or pink mopheads) is the tricky one, and big leaf hydrangea pruning mostly means not pruning. Older varieties bloom only on last year's stems, and Iowa State notes they often die back to the ground in Iowa winters. Newer reblooming varieties also flower on new growth. Wait until leaves emerge in spring, then cut out only the dead stems.
• Oakleaf hydrangea blooms on old wood and needs little pruning. Trim right after flowering if at all.

What about cutting back hydrangeas in fall? Wisconsin Extension lists fall, once dormant, as an option for smooth hydrangea and a light fall trim for panicle types. Never cut a bigleaf down in fall.

Can I Trim My Lilac Bush in the Fall?

Not if you want flowers next spring. Lilacs set next year's flower buds the season before, and Illinois Extension warns that pruning spring bloomers in late summer, fall or winter removes those buds. Trim your lilac as soon as the flowers fade. In fall, limit yourself to dead or broken branches.

An old lilac that has grown into a tree-shaped tangle can be renewed by removing a third of the oldest stems each year for three years. Cutting every stem to the ground also works, but Illinois Extension notes a lilac may not bloom for several years afterward.

Month-by-Month Shrub Calendar for Northern Illinois

Fall is still busy, just not for pruning. It is the season for leaves, bed cleanup and fall cleanup. For the lawn side of the year, see our Illinois lawn maintenance schedule.

Bringing Back Overgrown Shrubs: Renewal and Hard Cutbacks

Builder foundation plantings that went in 20 or 30 years ago are often over the windows now. Most deciduous shrubs can be brought back, and the timing for big cuts is late winter or early spring.

• Renewal pruning (the one-third rule). Each year, cut about a third of the oldest, thickest stems at ground level. With 12 stems, that is four a year for three years, and by the end no stem is more than three years old. You keep a shrub in the bed the whole time.
• Rejuvenation (the hard cutback). Cut every stem to 4 to 6 inches in early spring, then thin the new shoots the next year. Illinois Extension names red-twig dogwood, spirea and flowering quince as good candidates; Purdue adds forsythia, rose of sharon, privet and lilac. Expect fewer flowers for a few years.
• Evergreens are different. Yews can be cut back hard. Junipers and arborvitae cannot: cut into the brown interior and it stays bare.

Some shrubs are past saving: hollow arborvitae, junipers that are mostly dead wood, or a shrub that will always outgrow its spot. That is when bush removal and a bed renovation with plants sized for the space make more sense than another cutback.

How to Trim Hedges So They Stay Full

• Wider at the bottom. Penn State Extension recommends shaping hedges like an inverted V so sunlight reaches the lower branches. A hedge wider on top goes bare at the base.
• Shears for hedges only. Illinois Extension says most shrubs should not be sheared; save it for formal hedges like yew, boxwood and arborvitae. Everything else gets hand-pruner cuts.
• Open them up. Closely sheared yews need thinning every few years so light reaches the center.

Rather not own a hedge trimmer? Bush and hedge trimming is quoted free from a photo, with clippings hauled away.

Do Not Do This: Shrub Pruning Mistakes

• Pruning spring bloomers in fall or winter. You are cutting off next spring's flowers.
• A heavy trim in August or September. Tender regrowth goes into winter unhardened.
• Shearing every shrub into a ball. It thickens the shell and leaves a bare interior.
• Cutting arborvitae or juniper into brown wood. It will not fill back in.
• Cutting bigleaf hydrangea to the ground in fall. Wait for spring leaves and remove dead stems only.
• Pruning yews after July. Late growth gets damaged in winter.
• Shaking snow or ice off frozen branches. Morton notes cold wood is brittle. Let it melt, then prune any breaks.
• Carrying disease shrub to shrub. Illinois Extension suggests cleaning blades between plants with 70% isopropyl alcohol or a 10% bleach solution.

Frequently Asked Questions

Sources

• University of Illinois Extension: Dormant Season Tree and Shrub Pruning
• University of Illinois Extension: How to prune flowering shrubs for more blooms
• University of Illinois Extension: How to properly prune hydrangeas
• The Morton Arboretum: Pruning Deciduous Shrubs by species
• The Morton Arboretum: How to Prune Evergreen Trees and Shrubs
• The Morton Arboretum: Eastern arborvitae
• The Morton Arboretum: Help Plants Handle Winter
• Purdue Extension: Some Shrubs Best Pruned After Flowering
• Purdue Extension: Renewal pruning has immediate, delayed rewards
• Iowa State Extension: When should I prune my shrubs?
• Iowa State Extension: How should I prune my hydrangeas?
• University of Wisconsin Extension: Deciduous Shrub Pruning Chart
• Penn State Extension: Pruning Needled Evergreens
• Chicago Botanic Garden: Pruning Evergreen Hedges
• Illinois State Climatologist: New USDA plant hardiness zones map