Stump grindings are good for gardens as a mulch on paths and around shrubs, trees and perennials, but they should not be dug into vegetable or flower beds while fresh. Keep black walnut grindings away from tomatoes and other sensitive plants, and compost the rest first if you want it in the soil.
What you are actually getting
Grindings are not bagged mulch. They are shredded stump wood, bark and root mixed with the soil the stump stood in, and any weed seed in that soil comes along. The size runs from sawdust to thumb-sized chunks and stringy root. In a garden that mix has two lives. Spread on top of the ground, it is a free, decent mulch that holds moisture, cools the soil and smothers weeds. Mixed into the soil while fresh, it is wood competing with your plants for nitrogen. Nearly every piece of advice about grindings in gardens comes down to that difference.
On top: fine. Dug in: not yet.
University of California advisers, reviewing the research on wood chip mulch, say chips on the surface have no effect on the nitrogen in the soil below, and only draw it down when they are worked into the soil. They also say wood chips do not change soil pH, despite the common worry. Clemson Extension adds a caution for small chips, which break down faster and can use some soil nitrogen, so feed nearby plants if they pale. The practical rule is simple. Leave fresh grindings on the surface. If you want them in the soil, compost them first and dig in the finished, crumbly result.
In the vegetable garden
Use grindings on the paths, not in the rows. A thick layer on walkways between raised beds or rows keeps mud off your boots and weeds down, and it can be topped up each year. Around the crops themselves, use something finer. Clemson Extension's mulch guidance points to straw for the vegetable garden. Do not till fresh grindings into a vegetable bed in spring, because the crop will be short of nitrogen while the wood rots. Composted grindings are a different matter and can go in like any other compost.
Black walnut is the exception
If the stump was a black walnut, keep its grindings out of the vegetable garden altogether. Purdue Extension explains that juglone, a natural chemical found in all parts of the tree and especially the roots, harms many plants, and states that walnut bark and wood chips should not be used to mulch sensitive plants. Its list of sensitive vegetables includes tomato, pepper, potato, eggplant, cabbage, asparagus and rhubarb. Purdue also notes that composting may not remove all of the risk. So tell the operator it was a walnut, and have those grindings hauled or spread well away from beds.
Grindings from a diseased tree
University of California advisers say chip mulch does not pass disease to healthy roots, and that infection from diseased chips worked into soil is rare in the field. One group of fungi deserves care all the same. University of Minnesota Extension notes that Armillaria root rot, the honey fungus, lives on for years in dead stumps and roots. When a root rot killed the old tree, the safe move is to keep its grindings away from new plantings, especially trees and shrubs, and ask your county Extension office before using them in beds.
Around shrubs, trees and flower beds
These are where grindings earn their keep. Clemson Extension's guidance is a layer of two to three inches, held a couple of inches clear of shrub stems and tree trunks, and six to twelve inches back from the walls of the house. Do not heap them around a trunk. Because grindings carry soil and weed seed, many gardeners use them as a base layer and top them with an inch of cleaner mulch for looks. Expect them to sink and fade to grey within a year or so, and to need topping up.
Composting grindings for the soil
To turn grindings into something you can dig in, compost them. Layer them with green, nitrogen-rich material such as grass clippings or vegetable scraps, wet the mix to the feel of a wrung-out sponge, and build a pile at least three feet across, as NC State Extension suggests. Turn it when the middle cools. The finished product is dark and crumbly. How long stump grindings take to break down covers the timing, and what to do with stump grindings covers the other uses.
Common Questions
Can I use stump grindings in my vegetable garden?
On the paths, yes. In the beds, only after they are composted. Fresh wood dug into soil ties up nitrogen the crops need. Never use black walnut grindings near vegetables: Purdue Extension lists tomato, pepper and potato among the plants harmed by walnut's juglone.
Do stump grindings make soil acidic?
No. University of California advisers reviewing the research on wood chip mulch say the chips do not change soil pH. Any acidity stays at the thin line where mulch meets soil and is quickly neutralized. Test the soil if you are worried about pH.
Are black walnut stump grindings safe to use as mulch?
Not around sensitive plants. Purdue Extension says walnut bark and wood chips should not be used to mulch them, and that composting may not remove all the juglone. Use walnut grindings well away from vegetable and flower beds, or have them hauled off.
What to Do Next
Want the grindings left in a pile for the garden, or hauled away? Say so when you ask for a price, and mention if the tree was a walnut. The number is (828) 574-4474, and how the work runs is on the stump grinding page.