7 plants to never prune in September – a professional gardener's take on the flowers and shrubs not to trim (or risk killing them)

Pruning is all about timing. Believe me, I’ve learned this the hard way. Cut at the wrong moment, and you can ruin next year’s display, or worse, lose the plant altogether. Years ago, in a garden I was working in, I trimmed a lavender hedge a little too late in September. It looked perfectly neat, but the weather turned cold earlier than expected, and the tender new growth I’d triggered didn’t have time to harden off. A mistake I haven’t made since.

September is a tempting time to tidy – the summer’s slipping away, and the urge to neaten things up takes hold. But many plants, especially those that bloom on old wood like lilacs and forsythia, are best left alone. Prune them now, and you’ll be snipping off spring before it’s even had a chance.

So, while you may already know which plants to prune in September, here are seven plants I never cut back this month, and why, after a decade of gardening, I avoid any temptation to trim these species.

7 plants to never prune in September

I’ve worked with enough gardeners (and made enough errors myself) to know that once the secateurs are out, it’s easy to go pruning mad. But not every plant wants a haircut right now.

Before you start, then, take a pause and brush up on common pruning mistakes, so you don’t accidentally set your garden back a whole season. Here are seven plants to never prune in September.

1. Oakleaf hydrangea

I have grown oakleaf hydrangeas, or Hydrangea quercifolia, in several gardens where I have worked, and I remain a little obsessed. With oak-shaped leaves complemented by the white flowers in springtime, this shrub is quite a sight to behold.

But don’t be tempted to trim now. Oakleaf hydrangeas flower on old wood, which means this year’s stems are already forming next year’s show. Snip those off in September and, well, you’ll be met with a whole lot of nothing next spring.

Instead, in terms of when to prune an oakleaf hydrangea, I would suggest waiting until after it flowers, late spring or early summer, and then trim lightly.

If you must tidy, simply deadhead by snipping only the brown flowers, using something like these highly rated Felco F2 pruners, available to order via Amazon.

2. Lilacs

Lilacs are another shrub to never hard prune in September or during fall. Like many spring bloomers, they form next year’s flower buds in the summer months, well before the leaves drop.

If your lilac looks messy, you can remove the odd bit of dead wood or wayward stem. For thicker branches, try this cordless electric hedge trimmer by Ryobi, available at Home Depot now, which is highly rated by shoppers.

But in terms of when to prune lilacs properly, save this job for right after flowering in spring. That’s your window. Anything later and you'll be cutting flowering stems for next year.

If you fancy treating yourself to a new lilac, fall is a good time to get planting. This compact 'Bloomerang' variety, with live plants available from Proven Winners via Amazon, would be a good choice for smaller plots or patio plants.

3. Veronicastrum

I love darkened seed heads in the fall garden, and Veronicastrum virginicum, or Culver's root, might be the best.

This native plant (hardy down to zone 3) produces impactful blackened seed heads, as can be seen in the image above, that can stand for much of fall and winter, until they are flattened by snow.

So, my advice? Don’t cut these plants now. The faded flowers feed wildlife, birds pick away at the seeds, and pollinators still hover if the weather’s mild.

The same goes for coneflowers and black eyed Susan plants, let the cones and domes blacken, dry and stand tall to provide winter interest, habitat and food.

I would consider these as plants to prune in spring, so save the haircut for February or March, once new shoots begin to emerge from the base. Until then, enjoy the decay.

If you like the look of this perennial but don't have it in your yard, order Veronicastrum virginicum seeds from Amazon now, and scatter the seeds outside during September and October. With any luck, these seeds will germinate come spring and grow into healthy plants.

4. Switchgrass

I would also consider most ornamental grasses as plants to never prune in September. This would include one of the best native grasses, Panicum virgatum, otherwise known as switchgrass.

Live switchgrass starter plants can be ordered now via Amazon.

While they lose their lushness during September and October, and begin to bronze and brown, this is to be celebrated. To me, grasses are some of the most underrated elements in the fall and winter garden.

Cutting it back in September robs you of all that fall color and impact. Plus, these grasses act as habitat. Birds nestle in and insects overwinter in dense thickets. So, put the loppers back in the shed, and instead, cut back ornamental grasses in February.

5. Forsythia

Forsythias only bloom on old wood, so the stems that grew this year are the ones that’ll produce zingy yellow flowers next spring.

So, if you’re tempted to tame your forsythia now, resist the urge. In terms of when is it too late to prune forsythia, September is definitely too late for my liking.

You want to chop just after the blooms drop in late spring or early summer. That gives the shrub time to regrow fresh flowering wood through the rest of the year.

If you want to, you can remove any dead or diseased stems now, using something like these heavy-duty loppers from Fiskars, available online now at Amazon, which will easily cut through larger, rotten stems.

6. Penstemon

I’ve grown all sorts of penstemon over the years, from the deep purple ‘Raven’ to the fine-foliaged Penstemon pinifolius, which has the most striking fiery orange flowers.

These perennials always deliver. But a word of caution: don’t cut them back in summer or fall. I leave the flower stalks standing, and not just for the seed. Those hollow stems offer shelter for overwintering insects and provide food during the hummingbird migration – remember that even brown flowers can still contain nectar.

So, while you might be tempted to deadhead penstemon in September, I would suggest leaving them to stand. Instead, trim them down to the base in early spring, just when you can see where the new growth is pushing through.

7. Crepe myrtle

Crape myrtles are also plants to never prune in September. This is because a trim now will encourage soft new growth to shoot, which is the last thing you want going into fall and winter. An early frost in New York or Chicago, for example, would easily kill this growth.

So, in terms of when to prune crepe myrtle, be patient; you want to wait until late winter or very early spring when the shrub is dormant, just before any lush new leaves begin to unfurl. That way, you’re not encouraging risky new shoots.

When it comes to pruning, if there are any out-of-reach crepe myrtle stems, use something like these highly rated Fiskars extendable pruners, available now via Amazon.

One plant that you can trim in early September is lavender. Nobody wants to see woody lavender plants in their yard, so a good late summer chop is needed. I aim to trim in August or early September, giving enough time for any new growth to harden before frost. And remember, don't cut into old wood, only into the green growth.

For more inspiration and jobs to keep you busy this month, see our guide on perennials to plant in September for spring blooms.

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