Tree Trimmings

How would you like to transform your soil from compacted concrete (or lifeless sand) for free?

​You can, with tree trimmings!

Unprotected, compacted soil

Rich, loose soil under wood chip mulch

​Tree trimmings, also known as arborist chips or Tahoe chips, consist of coarse, medium, and fine wood chips, pine needles, twigs, leaves, and bark. It isn’t the pretty, uniform stuff you buy in bags from the garden center. This is what’s left over after a hard day’s work of trimming trees.

Professional arborists chip everything they cut off a tree into bits. These tree trimmings either go to a landfill or civic composting center, for a fee, or to someone’s driveway, for free.

A mountain of free wood chips!

Benefits of mulching with tree trimmings

Tree trimmings make an ideal mulch. The green leaves and new buds break down quickly, while medium-sized twigs and stems break down a little more slowly. Coarser pieces take longer to breakdown. This process creates a time-release of nutrients and organic material that provide a wealth of benefits, including:

• improved soil structure
• food for plants and valuable soil microorganisms
• reduced soil compaction and erosion
• soil temperature stabilization
• moisture retention
• curtailed weed growth

Dark, moist, rich soil created and protected under mulch of tree trimmings

But, what about toxins or disease? Let’s learn the truth.

Toxic wood chips

You have probably heard rumors about the way certain types of wood chips are toxic to garden plants, preventing germination or killing them outright. In nature, this form of chemical warfare is called allelopathy and black walnut is usually the first to come to mind. It is true that black walnut trees produce a compound, called juglone, that can kill off competitors. But juglone only affects some shallow-rooted plants and black walnut is too valuable as lumber to end up chipped. Other commonly accused chips include cedar, redwood, and Douglas fir, but there has been zero scientific proof that trimmings from these trees negatively affects other plants. They do, however, inhibit some fungi and bacteria, and repel or kill clothes moths, termites, cockroaches, ants, and carpet beetles. 

What to look out for

Before accepting a load of wood chips from your local arborist, be sure to ask if the load contains any palm tree trimmings. New palm trees can grow from these pieces. This is not what you want in your garden. Believe me. Here in California, we call palm trees ‘rat hotels’ for good reason. If the load contains palm tree, let it go somewhere else.

Many people are concerned about diseases and pests being transferred through tree trimming loads. For the most part, this fear is unwarranted. In nearly all studies, pathogens such as Verticillium, Cytospora, Thyronectria, were found unable to move from the mulch layer, into and through the soil to plant roots. They simply couldn’t do it. So don’t worry about tree trimmings bringing disease to your landscape, unless you make the mistake of digging the chips into the soil. It is far better to just dump them on top of the soil and let nature do all the work.

[Note, most pathogens are already present. They become a problem when plants become unhealthy enough to be susceptible. Mulching with tree trimmings helps your soil and plants be healthier, and less prone to disease.]

Speaking of work, moving wheelbarrows full of tree trimmings from your driveway to your landscape is work. It’s not backbreaking, but be sure to wear sunscreen, a hat, and gloves. You can skip the gym.

Wheelbarrow filled with tree trimmings

Tree trimmings and nitrogen loss

As mulch of any type breaks down, the microorganisms responsible for decomposition consume nitrogen. This makes nitrogen temporarily less available to nearby plants for 6 to 8 weeks. This is only true for the most shallow-rooted plants and has no effect on deep-rooted, established plants. You may need to add a little extra nitrogen during this time if you apply tree trimmings near vegetable plants. In the long term, as the wood continues to break down, it will add nitrogen and other nutrients to the soil.

And if you happen to see white fungal threads spreading through your tree trimmings, fear not! These fungi are a blessing to your soil and should be celebrated for the good health they bring to your plants.

Tree trimmings and fire safety

Some mulches are more flammable than others. If you want to improve soil structure, retain moisture, and reduce weeds, tree trimmings are your best choice. A group of fire and gardening professionals got together in Nevada to test various types of mulch for fire safety. The material you are most likely to get from an arborist is rated with moderate flames and spread, compared to shredded rubber, which burns the hottest (630°F) and flames the highest (over 3 feet), and composted wood chips, which is the only material that did not “demonstrate active flaming combustion”. So, as your tree trimmings break down, they will go from low risk to no risk.

How to get tree trimmings

Most arborists are happy to give away full or partial truckloads of perfectly good mulch. Simply give them a call and let them know you are interested. They will put your name on a list and call you when they have a load available. Then, park your car somewhere else for a few days and get ready to transform your landscape!

And be sure to gift your friendly, neighborhood arborist with the fruits of your garden labor. You will want another load in a few years!

Spread those tree trimmings 4 to 6 inches thick and let the magic happen. As always, keep mulch several inches away from trunks and stems to avoid diseases, such as crown rot.

Now, go call your local arborist!