How to support birds and beneficial insects without sacrificing your Portland perennial garden

I just received a wonderful newsletter from our local, one-and-only Cornell Farm. I love that place—and the café, too!

They have a helpful article about leaving your leaves this fall and winter to support insects and other wildlife that help feed nesting birds next spring. I’ll include a link at the bottom of this post.

I wholeheartedly support this practice. With insect populations under pressure, we need to restore habitat and put more insects back into our local ecosystem. Birds need insects, especially caterpillars, to feed their growing babies.

But as a Portland landscape designer, I have an important caution to add: Where you leave your leaves matters just as much as whether you leave them.

There is a lot of quick, easy advice encouraging us to spread fallen leaves over garden beds instead of using bark chips or other wood-product mulches. The ecological intention is good, but not every garden—or every plant—is suited to a blanket of wet leaves all winter.

Many of our trees in the Pacific Northwest have large, heavy, and slow to dry leaves. Some break down relatively quickly, while others form a dense layer that stays wet for months. Depending on the surrounding plants and conditions, this heavy wet leaf layer can contribute to crown or root rot and provide shelter for slugs that feed on emerging shoots in spring.

We can support wildlife without sacrificing our plants. Here’s how I approach it.

1. Get to know your leaves

Different trees produce different kinds of leaves, and that makes a difference in how I use them in the garden.

Leaves that break down well

In my experience, leaves from ginkgo, crape myrtle, and dissected (lacy) Japanese maples work well and break down nicely, acting as a lighter layer of organic material. Leaves from native alder and black hawthorn are also small enough for me to use as mulch although more fibrous and slower to break down. So think about the size and thickness of leaves to use as mulch. I might be able to let some kinds of leaves lay where they fall. Even with these smaller leaves, I remove most of the accumulation off tops of evergreen shrubs and ferns rather than robbing the plants of sunlight all winter. Sometimes I can just bat at my shrub with my garden rake to knock off the leaves. Other times it requires hands on fiddling about to get enough leaves off the evergreen plant.

2. Be careful with perennial plants and pollinator garden beds and leaf mulch

This is where I want my clients to pause before following advice to leave every leaf in place.

You can’t see what’s happening beneath a thick layer of leaves. In our wet winters, they provide shelter for slugs, which lay eggs in fall and hatch to emerge in spring, just as perennials begin sending up new growth. When the new slugs hatch in late February or March they eat the shoots and young leaves of many of your plants out of sight where you cannot see what is happening.

Repeatedly losing new shoots to slugs can eventually kill a perennial, especially a young plant whose roots may not have enough stored energy to keep starting over.

I don’t use many slug-susceptible plants in my designs, but who would design a pollinator garden without coneflowers? I lost a Rudbeckia laciniata last spring after slugs repeatedly devoured its new shoots.

Other plants I use in my designs that are susceptible to either root rot or sneaky slug devastation from under the leaves include:

3. Protect plants that are sensitive to winter wet

Slugs aren’t the only concern. Some plants are susceptible to rot when their crowns, roots, or storage structures remain too wet.

Thick layers of heavy leaves can be particularly troublesome around dahlias, succulents, and some salvias. Dahlia tubers can rot in excessively wet conditions, many succulents are poorly suited to prolonged winter moisture, and some salvias need excellent drainage around their crowns and roots. My gramma tried mulching her large Dahlia bed with Oregon Big Leaf Maple leaves because her neighbor said to try it. She lost every single dahlia over that winter.  For those who eschew Dahlias, your species Echinacea are at risk of rotting from heavy leaves for mulch.

Our Portland leaves are not necessarily the light, fluffy leaves you might picture in a drier climate. Here, they often become saturated and matted down by months of rain.

If you have plants sensitive to winter wet, avoid piling leaves over their crowns or blocking drainage. A little attention in autumn and early winter can prevent a disappointing discovery in spring.

4. Where should you leave your leaves?

There are plenty of places where leaves can do their ecological work without putting your garden at risk.

• Under trees where little else grows. For example, beneath a large plum tree where the ground is mostly bare.
• In open areas between shrubs. These can be excellent places for leaf piles, provided they don’t smother the shrubs’ crowns.
• In sheltered areas along boundaries. These spots can keep leaves from blowing around the garden, but don’t pile them against house siding or where they could create moisture problems.
• Beneath trees in naturalized areas. This works especially well when the planting was designed with the tree’s own leaf litter in mind.
• In dedicated leaf piles. If you have room, undisturbed piles can provide overwintering habitat without covering your prized perennials.

Some designers who specialize in native plant gardens plan carefully for the way leaves will fall and accumulate. That’s a wonderful approach when the plant palette and layout are suited to it. Simply adding a thick layer of leaves to an established garden, however, doesn’t automatically make it a better wildlife garden.

As a landscape designer, I consider how a planting will perform throughout the year, not just in summer when it looks its best. That includes our long, wet winters.

5. You can help birds without losing your perennial pollinator garden

I encourage everyone to do their part for wildlife. We need more habitat for the insects that sustain birds and other creatures, and leaving appropriate areas undisturbed over winter is one practical way to help.

We don’t have to choose between supporting wildlife and protecting our gardens. Leave leaves in selected areas, maintain piles where they won’t smother plants, and preserve suitable leaf litter beneath trees and shrubs. Leave standing perennial stems where appropriate, too, as they can provide shelter for wildlife.