Hydrangeas: what to cut in fall and what to leave alone

You've raked the leaves, the mower is almost done for the year, and that hydrangea by the front steps looks ragged: brown flower heads, floppy stems, and leaves going yellow. You've got pruners in your hand and the urge to tidy it up before Halloween. For most hydrangeas in the northern US, don't do it. You can safely cut very little in fall. The useful work this time of year is figuring out which type you have, protecting the tender ones and saving the real pruning for spring.

The short answer: mostly leave it alone

Cutting hydrangeas in fall is the most common way people lose next year's flowers. Many homeowners prune in October to clean up, and then the plant comes up leafy and green in June with no blooms.

It comes down to timing. Bigleaf, oakleaf and mountain hydrangeas bloom on old wood, meaning stems that grew the year before. University of Minnesota Extension says bigleaf hydrangeas set next year's flower buds in August and September. By now those buds are already on the stems. If you cut the stems, you cut off next summer's flowers. UNH Extension says bigleaf hydrangeas should never be pruned in late summer or fall.

Panicle and smooth hydrangeas work differently. They bloom on new wood, the growth they put on next spring. A fall cut won't cost you flowers, and Minnesota Extension lists fall as an acceptable time to prune panicle types. Even so, they recommend waiting until late winter or spring, and so do I.

A golf course works the same way. The crew does the work when it pays off, not when the calendar makes them feel like doing something. On every hydrangea you own, fall pruning has a real downside and almost no upside.

Which hydrangea do you have?

Everything depends on the type, so start here. Illinois Extension sorts them into two groups.

Old wood (blooms on last year's stems; don't cut in fall):

• Bigleaf, mophead form: big round flower balls, often blue or pink, with large, shiny, coarse-toothed leaves. This is the classic hydrangea people lose blooms on.
• Bigleaf, lacecap form: same leaves and plant, but the flower is a flat disc of small fertile flowers ringed by larger showy ones.
• Oakleaf: the leaves are lobed like a big oak leaf. Clemson says it blooms on last year's wood, typically in June.
• Mountain: a close cousin of bigleaf, usually a smaller plant, also on old wood.
• Climbing: the vine that climbs walls and tree trunks. It's old wood too, and Minnesota Extension says it needs little to no pruning beyond taking out dead wood.

New wood (blooms on this year's growth; can handle a cut, but it can wait):

• Panicle: cone-shaped flower clusters, usually white and aging to pink or tan, on stiff upright stems.
• Smooth ('Annabelle' type): big round white heads on thinner stems that tend to flop. Minnesota Extension says its flower buds form on the current season's growth.

The in-between: Endless Summer and other rebloomers. These are bigleaf hydrangeas bred to bloom on both old and new wood. Minnesota Extension says they bloom whether or not you prune. Clemson notes that the first flush of flowers still comes from old wood and the later flowers come on new wood. If you cut one back in fall, you lose the early show.

The rule for everyone else: if you can't tell what you have, don't cut it. Leaving it uncut costs you nothing. A wrong cut can cost you a whole summer of flowers.

What you can cut in fall

The list is short. These are general good practice for any shrub, and they're safe on every hydrangea type:

• Dead stems. If a stem is clearly dead, brittle and gray with no buds, it isn't carrying anything for next year.
• Broken stems. Wind, snow-blower spray and the dog all snap hydrangea stems. Cut the break back cleanly.
• Crossing or rubbing stems. Where two stems grind against each other, remove the weaker one.
• Spent flower heads, if they bother you. On old-wood types, take only the dried flower and stop at the first set of fat buds below it. Don't cut lower than that.

On panicle and smooth types you have more freedom, but I'd still leave the flower heads on. Minnesota Extension recommends waiting until late winter or spring partly because the dried heads add winter interest. A panicle hydrangea with tan cones holding snow looks a lot better in January than a row of stubs. It also means one less job on your fall list.

Skip the hard cutback for now. It belongs in late winter or early spring, which I cover below. Fall is short enough already. Spend it on the lawn and leaves, and let the hydrangeas wait.

Winter protection for the old-wood types

Bigleaf hydrangeas are where northern gardeners get burned. The flower buds sit on the stems all winter, and they're more tender than the plant. UNH Extension says extremely low temperatures, wide temperature swings and late spring frosts all damage bigleaf buds. A plant killed to the ground rarely blooms that season. The roots usually survive and the plant regrows, but it grows only leaves that year.

Here's what you can do once the plant has dropped its leaves and gone dormant, usually in the stretch between Halloween and Thanksgiving:

1. Build a cage and fill it. UNH suggests wrapping the plant in burlap or chicken wire and filling the enclosure with leaves. Dry, shredded leaves from your own yard work well and cost nothing. Leave the protection in place until all danger of frost has passed in spring. Don't pull it off on the first warm week of April.
2. Mulch the crown. Pile mulch or shredded leaves over the base to give the lowest buds and the crown some extra insulation.
3. Guard against rabbits. Rabbits will chew bigleaf stems down over the winter, which takes the buds with them. Minnesota Extension recommends a ring of hardware cloth about 4 feet tall around bigleaf hydrangeas. That's taller than the 24-inch fence they suggest for most shrubs, because snow piles up and gives rabbits a step stool.

To be honest, UNH says this protection isn't always effective. You can do everything right and still lose the buds to a hard January or a frost in May. If a bigleaf has given you leaves and no flowers for several years running, the fix is usually the plant or the spot, not more burlap. Move it somewhere more sheltered, or replace it with a reblooming type or a panicle or smooth hydrangea. On a course, if a plant needs babysitting every winter just to survive, we swap it for one that suits the site. That's how you keep the beds looking good without spending every weekend on them, and it works at home too.

Spring: judge damage, then cut

Spring pruning starts with patience. UNH Extension says to wait until mid to late spring to prune bigleaf hydrangeas, when you can actually see what made it through winter. Some stems that look dead in March will leaf out by May.

To check a stem, do a scratch test. Scrape a little bark off with your thumbnail or a knife. Bright green underneath means that part is alive. Tan or brown means that part is dead. If the tip is brown, move down the stem and test again, because often only the top few inches died. Cut back to just above live buds and leave everything that's alive.

After that, prune by type:

• Panicle and smooth (new wood): prune in late winter or early spring, before new growth starts. Illinois Extension says these can lose up to one-third to one-half of the shrub's mass. Minnesota Extension says established smooth hydrangeas can be cut to the ground in spring or left unpruned if you want a bigger shrub. Give young ones a season or two first. On panicles, cut to just above a fat bud, about 1/4 inch above it.
• Oakleaf, mountain and climbing (old wood): prune right after they flower, and only lightly. Illinois Extension says they need little pruning.
• Bigleaf (old wood): in spring, cut out dead wood and nothing else. If you want to shape or shrink the plant, do it right after it blooms in summer. Minnesota Extension says to finish before August, because that's when the new buds start forming. Clemson says to remove no more than a third of a bigleaf's total growth.

So if you want one rule to take into spring for bigleaf hydrangeas: cut only dead wood, wait until after bloom to shape the plant, and never take more than a third. For now, hang the pruners back up and go rake.

Sources (6)

• University of Minnesota Extension: Pruning hydrangeas for best bloom
• University of Minnesota Extension: Pruning hydrangeas
• University of Illinois Extension: How to properly prune hydrangeas
• Clemson Cooperative Extension HGIC: Pruning hydrangeas
• UNH Extension: How do I protect my Endless Summer hydrangea over winter?
• Ask Extension: Checking whether a tree or shrub is alive (scratch test)

Photo: Hydrangea Autumn by Amber Ladley, CC0.