Mulching Perennials: When, How Much and What to Use

A layer of mulch is one of the simplest ways to make a perennial garden easier to care for. It smothers weeds, keeps the soil moist and cool through summer, protects roots from sudden temperature swings and, as organic mulches break down, feeds the soil life that keeps perennials healthy. A well-mulched bed also simply looks finished.

Mulch can also cause problems when it goes on at the wrong time, in the wrong amount or in the wrong place. Too much smothers roots, mulch piled against crowns invites rot and voles, and a thick layer spread too early in spring can delay perennials from coming up. The difference comes down to three decisions: when to mulch, how much to use and which material suits your plants.

Why Mulch Perennials

Mulch does several jobs at once, which is why it is worth the effort:

• Weed control. A layer of mulch blocks light from reaching weed seeds in the soil, so far fewer germinate, and the weeds that do come up are easier to pull.
• Moisture. Mulch slows evaporation from the soil surface. Colorado State University Extension reports that wood chip and bark mulch can cut irrigation needs in perennial beds by up to 50 percent.
• Soil temperature. Mulch keeps roots cooler in summer and buffers the soil against rapid freezing and thawing in winter.
• Soil health. Organic mulches break down into humus, improving soil structure and feeding earthworms and beneficial microbes.
• Erosion control. Mulch protects bare soil from heavy rain, crusting and runoff, especially on slopes.

When to Mulch Perennials

Timing matters as much as material. Mulch keeps the soil at whatever temperature it is when you apply it, which can help or hurt depending on the season.

Spring Mulching

The main mulching of the year happens in late spring. Wait until the soil has warmed and your perennials have emerged and are a few inches tall. Mulching cold, wet soil too early insulates it from the spring sun and can slow plants from coming up. Ideally, get the mulch down before summer weeds begin to germinate, which Virginia Cooperative Extension notes starts once soil temperatures reach about 50°F. In much of the northern United States and southern Canada, mid to late May is a good target.

Summer Top-Up

Fine materials such as compost, shredded leaves and grass clippings break down quickly in warm weather. If bare soil shows through by midsummer, add a thin layer to restore the depth, especially before a hot, dry spell.

Fall and Winter Mulch

A winter mulch has a different purpose from summer mulch: it keeps the soil frozen at a steady temperature so perennials are not heaved out of the ground by repeated thaws. Apply it after plants go dormant and the top couple of inches of soil have frozen, usually in late November or December in northern gardens. Mulching too early in fall can delay dormancy and give voles a warm place to nest. Shredded leaves, straw and evergreen boughs work best for winter protection.

How Much Mulch to Use

For most perennial beds, 2 to 3 inches of mulch is the right amount. Coarse materials such as shredded bark and wood chips can go a little deeper, while fine materials such as compost should be thinner, since they pack down and hold more water. Never let the total depth of mulch build up beyond about 4 inches. More is not better; a thick layer keeps air and water from reaching the roots and can cause shallow rooting and rot.

Mulch	Depth	Best For
Shredded hardwood bark	2 to 3 inches	General perennial beds
Shredded leaves or leaf mold	2 to 3 inches	Woodland and shade gardens, winter protection
Pine needles	2 to 3 inches	Beds that drain well, slopes, acid-loving plants
Compost	1 to 2 inches	Feeding the soil, newly planted beds
Arborist wood chips	3 to 4 inches	Paths and large, established plantings
Fine gravel	1 to 2 inches	Drought-tolerant and Mediterranean perennials
Straw	3 to 4 inches, loosely	Winter protection and new plantings

Around individual plants, taper the mulch down to an inch or less and leave a gap of about 2 inches around each crown. Virginia Cooperative Extension recommends never covering the crown itself.

How to Mulch Perennials

A few minutes of preparation makes mulch far more effective.

Weed First

Pull or dig out existing weeds before mulching, including their roots. Mulch smothers new weed seedlings, but established perennial weeds will simply grow through it.

Water the Soil

Mulch holds in whatever moisture is already there, so water dry soil thoroughly before spreading it. If the soil is already moist after rain, you can mulch right away.

Spread Evenly and Pull Back from Crowns

Spread the mulch evenly across the bed with your hands or a small rake. Around each perennial, pull it back to leave a small ring of bare soil at the crown, and never bury emerging shoots. Feather the layer down to an inch or less close to the plants.

Edge the Bed

A clean edge, cut with a spade or half-moon edger where the bed meets the lawn, keeps mulch from washing into the grass and grass from creeping into the bed.

Best Mulches for Perennials

The best mulch is one that breaks down slowly enough to last a season, lets water through easily and improves the soil over time. These materials all work well in perennial beds.

Shredded Hardwood Bark

The most widely used mulch for perennial borders, shredded bark knits together into a stable layer that resists washing away, breaks down slowly and gives beds a tidy, natural look. Double-shredded bark is finer and suits smaller plants.

Shredded Leaves and Leaf Mold

Shredded fall leaves are free, look natural and break down within a year into rich, crumbly soil. They are ideal for woodland and shade gardens, where they mimic the forest floor that plants like hostas, ferns and astilbe grow in naturally. Always shred leaves first; whole leaves mat into a soggy layer that sheds water and smothers crowns. Leaf mold, which is leaves that have decomposed for a year or two, is one of the best mulches for perennial borders.

Compost

Compost feeds the soil more than any other mulch, but it breaks down quickly and can allow weed seeds to germinate on its surface. Use it as a thin layer under a coarser mulch, or on its own in beds of heavy feeders.

Pine Needles

Pine needles, also called pine straw, form a light, airy layer that stays in place on slopes, lets water through freely and lasts a season or more. Despite a common belief, they have little effect on soil acidity as they break down.

Arborist Wood Chips

Wood chips from tree services are often free and contain a mix of wood, bark and leaves. They last a long time and are excellent for paths and large beds of established perennials. Their chunky texture can overwhelm small plants, so use finer mulches around them.

Gravel

Fine gravel suits drought-tolerant perennials that dislike moisture around their crowns, such as lavender, Russian sage, sedum and many salvias. It keeps the crowns dry, suppresses weeds and reduces evaporation, although it adds more heat to the soil than organic mulches and does not improve the soil.

Mulches to Use with Care

Some materials cause more problems than they solve in perennial beds.

• Fresh grass clippings mat down into a slimy, water-shedding layer when applied thickly. Use them only in thin, dry layers.
• Hay is full of weed and grass seeds. Use straw instead.
• Whole, unshredded leaves smother crowns and shed water.
• Landscape fabric and plastic sheeting reduce air and water exchange, clog with soil over time, trap roots and prevent perennials from spreading and self-seeding. Colorado State University Extension does not recommend them for garden beds.
• Rubber mulch lasts for years but adds nothing to the soil and is difficult to remove.

Does Wood Mulch Take Nitrogen from the Soil?

Many gardeners worry that wood mulch robs perennials of nitrogen. The concern is mostly misplaced. When wood decomposes, soil microbes use nitrogen to break it down, but this happens mainly where the wood touches the soil, not in the root zone below. Colorado State University Extension notes that wood chip and bark mulch spread on the surface do not tie up soil nitrogen. The problem arises when fresh wood chips or sawdust are mixed into the soil, which can cause a nitrogen shortage for years. Keep wood mulches on top of the soil, and use aged or composted wood products around small or newly planted perennials if you are concerned.

Perennials That Need Little or No Mulch

Not every perennial benefits from a thick organic mulch. Drought-tolerant perennials from dry climates, including lavender, Russian sage, sedum, yarrow and many salvias, prefer lean, dry soil around their crowns; mulch them with gravel or not at all. Shallow-rooted plants with crowns at the soil surface, such as coral bells and creeping phlox, need only a light layer kept well away from the crown.

If you want perennials to self-sow, such as columbine, black-eyed Susan, foxglove and many native wildflowers, keep mulch thin or leave some bare soil, since a thick layer prevents seeds from reaching the soil and germinating.

Common Mulching Mistakes

These are the mistakes that cause the most problems in perennial beds:

• Piling mulch against crowns and stems, which keeps them wet and invites rot, slugs and voles.
• Letting mulch build up year after year until it is 5 or 6 inches deep.
• Mulching cold, wet soil too early in spring, which delays emergence.
• Spreading winter mulch before the ground freezes.
• Mulching over weeds instead of removing them first.
• Mixing fresh wood chips into the soil instead of leaving them on top.
• Using landscape fabric under mulch in perennial beds.

How Often to Replace Mulch

Organic mulch breaks down and needs topping up, usually once a year in spring. Check the depth before adding more: if an inch or two of old mulch remains, add just enough to bring it back to 2 to 3 inches. There is no need to remove old mulch that has broken down, since it has become part of the soil. Rake out or remove old mulch only if it has built up too deep, matted into a crust or become moldy, or if it is covering the crowns of perennials. Fine mulches such as compost and leaves may need a top-up in midsummer as well.

Frequently Asked Questions

These are the questions gardeners ask most about mulching perennials.

How Thick Should Mulch Be in a Perennial Bed?

Two to 3 inches is right for most perennial beds, thinning to an inch or less near each plant. Never let mulch build up beyond about 4 inches in total.

When Is the Best Time to Mulch Perennials?

Late spring, after the soil has warmed and perennials have emerged, usually mid to late May in northern gardens. Winter mulch goes on in late fall, after the ground has frozen.

Should You Remove Old Mulch before Adding New?

Usually not. Old mulch that has broken down improves the soil. Remove it only if it is too deep, matted, moldy or covering plant crowns.

Can You Use Leaves as Mulch for Perennials?

Yes. Shredded leaves make an excellent free mulch, especially in shade gardens. Shred them first, since whole leaves mat down and smother plants.

Should You Mulch around Perennials in the Fall?

You can add a winter mulch in fall, but wait until the ground has frozen. Mulching too early keeps the soil warm, delays dormancy and gives voles a place to nest.