NRCS Soil Health Principle #4: Keep it covered as much as possible.
OK. I’ll put some mulch on it. Wait… There are so many mulches to choose from. Which is the right kind? How much? Do I need to cover all my bare soil? Always? 😬🤯
Mulch choices are infinite. Literally anything can be packaged and labeled mulch. Here is a breakdown of what I’ve learned from training and experience, so you can have some basic knowledge to experiment with and find out what works for you.
Which Mulch is Best for… (in alpha order)
Pathways and Large Areas:
- Cedar Chips
- Chipped Hardwood
- Conifer Bark Nuggets
- Hog Fuel
Under Trees and Shrubs:
- Finely Shredded Hardwood
- Finely Shredded Conifer Bark
- Pine Shavings (Typically sold as pet and livestock bedding.)
Annual Garden Vegetables:
- Black Plastic
- Leaf Mold
Garden Bed Winterizing:
- Animal Manure
- Fall Leaves
Soil Enriching Green Manures:
- Grass Clippings
- Shredded Dynamic Accumulators (comfrey, artichoke)
Living Mulches in your Fruit Tree Guilds:
- Good choices are: sweet alyssum, comfrey, calendula, yarrow, thyme, oregano.
- Fall Leaves
Establish New Planting Areas with Sheet Mulch:
- Fall Leaves
Just Say No:
- Dyed Wood Chips
- Shredded Tires
Wood mulches are great for pathways and under trees and shrubs. They supress weeds, hold water in the soil and also keep it cool during the heat of the day and warm during the chilly nights. Wood mulches also break down slowly and add organic matter to the soil.They are carbon heavy, so as they break down they remove nitrogen from the top layers of soil. This doesn’t bother trees and shrubs with long vigorous root systems, but can be growth prohibitive for shallow rooted annuals.
Wood Mulches for Pathways and Large Areas
The more foot traffic and area receives the larger I want the mulch pieces to be so they last a long time, don’t end up in my shoes, and don’t become matted down.
Burlap Bags: Make wonderful paths between rows in large gardens. I saw this demonstrated at PCC Rock Creek’s student gardens, but I have never found a good source of free burlap to try it myself.
Cedar Chips: They smell wonderful and are a good choice for pathways and high traffic areas. I only need to refresh every three years or so. The aromatics of cedar chips repel insects, which is something to keep in mind if you want more insect activity in your yard. I find this effect to be temporary. After a few weeks, when the chips are not so fragrant the insects are all back. Still, I feel mildly guilty when spreading them.
Conifer Bark Nuggets: Large pieces of conifer bark hang around a while before breaking down. These have a beautiful red color. I chose these for my pathways because they shouldn’t repel insects like the cedar chips.
Hog Fuel: This is usually inexpensive, roughly chipped wood. It lasts a long time and is great for covering areas between beds and paths and other areas that get heavy foot traffic.
Under Trees and Shrubs
Finely Shredded Conifer Bark: This stuff is perfect for acid loving plants (rhododendrons, azaleas, camellias, and Western Oregon natives) to keep the pH low. This mulch can be splintery.
Finely Shredded Hardwood: I use this under my apple tree guilds, and under other trees, shrubs, ferns, and perennial flowers on our property. It looks nice, is not very splintery, and is light and fluffy.
Pine Shavings: Blueberries love up to six inches of fluffy pine shavings. This is the one and only time when mulch is allowed to contact the stems or trunks.
Sawdust: Conifer sawdust also works well.
Annual Garden Vegetables
Compost: Compost is also wonderful as a garden mulch. It holds in moisture, enriches the soil with phosphorus and potassium, (The PK of NPK) and adds organic matter. Its dark color helps the soil warm up in the spring. However, it isn’t great at weed suppression (and can have it’s own weed seeds sometimes). Also, it isn’t a great source of nitrogen. If you’re buying your compost it’s important to make sure it is tested against pyridine carboxylic acids that can damage your crops. Recology routinely tests their compost. Many other landscape supplies offer organic compost as well. Caution: Do not use compost on acid loving plants, blueberries, et. al. as the salts in the compost will raise the pH. I’ve made this mistake myself. 😬
Leaf Mold: Wonderful stuff. It holds an incredible amount of water and is warming and nutritious for the soil. I’ve never seen it for sale, so if you want some of this “black gold” for yourself, you’ll have to make it. I have a wire leaf bin I made and I collect all of our leaves as well as from my neighbors. It takes about two years to break down into leaf mold. I lift off the frame and under the dry leaves is this amazing crumbly earthy smelling mulch. Another method for making leaf mold is to use black plastic bags. Stuff them full of leaves, pierce them with holes and forget about them for about two years. My only problem is that I can’t seem to make enough. A leaf never leaves my property, if I can help it. (Except fruit trees. Pests and disease overwinter in fruit leaves. Those ones go in the bin.)
Straw: Untreated straw is my favorite mulch for garden beds, strawberries, and cane fruits. It’s light and fluffy, allowing for circulation of water and air. It almost always has a few seeds that germinate. I just pull them up and lay them down for more mulch. If I’m really on top of my game, I’ll add compost as mulch early in the season, and then straw in June.
Caution: Cereal crops are often treated with a class of chemicals called pyridine carboxylic acids that kill broadleaf plants. These chemical residues stay effective for about two years and can damage your garden plants.
Black Plastic is for Heat Loving Annuals: (Sweet Potatoes, 🌶️, 🍉, 🍅, 🍆, etc.)
It retains moisture, suppresses weeds, but is mainly used for gathering solar energy. It is usually purchased in a roll that is unrolled on the ground with the edges buried in soil. Then a hole is cut for the plants. Caution: Voles will use this for cover and their population explosion could put a serious dent in your crops.
Caution: Wait to Apply Mulches to your Garden Beds Until June: (Except the black plastic. Do that in May.)
Even though the principle is, “Keep it covered as much as possible”, be aware that slugs and sow bugs thrive in cool, moist soil that is rich in organic matter. They need places to hide from the sun during the day. Prep your annual beds in March and remove all mulch materials, (except compost). Then after your plants are established and the soil is warm (May or June) apply the mulch of your choice to keep the soil from getting too hot and dry.
Soil Enriching Green Manures
Comfrey, artichoke, and yarrow are good examples dynamic accumulators. They put out lots of foliage that is rich in nitrogen and minerals dredged up by their deep root systems. Comfrey can be cut two or three times a season. Chop and drop the leaves anywhere plants need a nutrient boost. For artichoke leaves, I only use the dropping lower leaves and chop and drop. Yarrow is for smaller spaces.
Grass clippings are also nitrogen rich and break down quickly. However, I usually run my mower on mulch settings and rarely have clippings to share with the garden. Sometimes I’ll add the clippings from the last mowing of the season on top of my winterized beds to boost decomposition.
Living Mulches in your Fruit Tree Guilds
As the Native Americans taught us with their three sisters guild, plants can do a great job of suppressing weeds while providing another harvest. In my fruit tree guilds, I use sweet alyssum, comfrey, calendula flowers, yarrow, thyme, oregano and other plants (mostly flowers) to crowd out weeds, cover the soil, feed the soil microbiology with their root exudates, repel pests, and attract beneficial insects.They don’t compete with the tree’s root system because they inhabit different rhizospheres and they are not nutrient or water hungry like turf grass.
Fall Leaves: When preparing beds for winter, I use a thick layer of fall leaves weighted down with fresh compost. I used to cover that with another layer of leaves but found that they just blew away. The worms and bugs will work hard for you all winter breaking down organic matter and pulling it deep into your garden soil. Remove any leftover leaves in the spring to encourage the bugs to die back and slugs to move out. My backyard orchard gets the same treatment.
Composted Manure: Instead of compost, manure is also wonderful in this application. Giving it all winter to break down will ensure that it is safe for planting in the spring. The fall is the only time I would apply fresh manure. Caution: Again ensure that the animals you received it from did not eat herbicide treated hay. Those pyridine carboxylic acids will pass through an animal and remain active in the manure.
Establishing New Planting Areas with Sheet Mulch
Rather than removing turf and or weed overgrowth by hand try sheet mulching. Fall is the best time to start this process. The second best time is now. Just wait to begin planting as this big pile of sheet mulch will breakdown to pretty much ground level.
Cardboard: When putting in new raised beds, use several layers of dampened cardboard over turf grass. Stab holes with your digging fork to permit gas exchange and promote worm movement. My first four beds I dug up the turf by hand. My jaw dropped when I learned this method, it has worked beautifully for me ever since.
Layers of Newspaper or Fall Leaves can be substituted for cardboard, but they are not quite as effective for smothering weeds.
Lasagna Mulching for New In-Ground Beds: In the fall, start with a layer or two of cardboard, (wet and perforated), then add a layer of compost, then a layer of leaves or newspaper, and lastly more compost or green manure. It’s a lasagna that has soil fauna saying YUMMY! Let them dine on it all fall and winter and even the heaviest clay soil (like mine) will begin to perform like great soil.
Dyed Wood Mulch: The artificial color bleeds out over time and looks bad. I always wonder what’s in the dye.
Shredded Tires: I love the recycling aspect, but vulcanized rubber is such a toxic mess. On a warm day it smells like the local tire center. It doesn’t decompose and to get rid of it you’d have to dig it up and throw it out. Your soil needs organic matter and the fact that most other mulches decompose is a good thing as it breaks down and feeds your soil microbes. Here’s a cheeky article on the subject: Just Say No to Rubber Mulch
When to Leave Your Soil Bare (Rules are made for breaking!)
- For our bee and bird friends. Mason bees appreciate access to clay soil. If you put up a mason bee house a small hole with exposed clay will be appreciated.
- Some birds build nests with clay and they appreciate access to clay soil.
- Ground bees also can’t dig through mulch to build their ground burrow so these guys also like exposed soil.
- Spring garden bed prep: Leave your garden bed soil uncovered to dry out, warm up, and encourage the slugs and bugs to move out. This bears repeating as I didn’t do this once and lost all my transplants to slug and pill bugs (sow bugs, potato bugs, po-tah-toe bugs). Only once your plants are established, past that delicate seedling stage, and the weather is beginning to get hot and dry, then you can add your favorite garden mulch.
It’s your garden. Your personal laboratory. If you have a lot of wood and want to try it in your vegetable beds, do it and see what happens. A lot of people use wood mulch this way and it works great for them. If your veggies start turning yellow or stay small, then take it off and try something else. Being a bit rebellious, I always have to test these theories out for myself and see. I always learn something unexpected and interesting.
Resources & Further Reading
- Want some free wood chips? Like a lot? Check out ChipDrop.
- NRCS Soil Health Primer (.pdf)
- Herbicide Carryover in Hay, Manure, and Compost and Grassclippings