City leaves often contain heavy metals, so compost with care.
A few years ago, I heard Maine farmer Will Bonsall “go off” in praise of leaves as a natural and free form of compost, better, he claims, than almost anything else you can buy in a bag. But he was talking about leaves acquired on or near his rural farm, not the ones raked off of city lawns.
I’ve been using about 100 bags of leaves as compost every season, collected from curbsides around Portland, where the locals conveniently rake it up and stuff it into paper, biodegradable bags.
What could be wrong with free, easy to access (often pre-shredded) leaves? Unfortunately, there’s a catch. (Isn’t there always?)
While reading an obscure book written in 1870 praising the use of “muck” by southern farmers (their term for half-rotted leaf mold), it ocurred to me that use of leaves in rural and urban settings shouldn’t be assumed to be equally virtuous.
The Lead Problem
In many areas of Portland, for example lead levels in soils are hundreds of times higher than “safe” levels, as determined by the EPA. I know this from samples I have had tested. Certain areas are worse than others, of course. Prior to 1978, most homes were painted repeatedly with lead-based coatings. Any house in Portland older than that likely STILL has lead content in the paint, the plumbing or elsewhere.
Lead doesn’t dissipate much on its own, so our soils are heavily afflicted with it. The older and denser the region of town, the higher the likely lead (Pb) content.
Back to my point about using leaves as mulch. Are they safe? Do they pick up heavy metals such as lead?
The answer, according to a study done in New Jersey on municipal leaf mulch, is yes, (unfortunately).
Researchers found numerous metals in curbside leaves, including lead, iron and aluminum. They suggest that this occurs because the fallen leaves “become contaminated with urban soil and dust from the road surface through various processes (raking, lawnmower pickup and vacuum) used for litter collection in the yard and at curbside.”
Of the various contaminants, lead is arguably the most concerning. How bad is the potential pollution in city leaves? They apply a standard of 45 Mg/ha, based on tests of sewage sludge–which equates–by my calculation, to about 45 tons of leaf matter spread on 2.5 acres of land.
Break that down to the garden plot level. An acre is 43,560 sq. ft. If you were only adding this much material to your beds, you would add about 1.21 lbs/sq. ft of leaf compost per year (108,900 sq. ft, divided by 45 tons/90,000 lbs.=1.21 lbs. per square foot).
Given this level of application, lead levels in your garden would not exceed the EPA rules for “annual pollutant loading rate”–at least for a few years. In the worst case scenario, with the highest level of lead contamination the researchers found in leaves, an application of this much leaf matter would take 16 years of repeated use to exceed EPA levels.
Don’t get too relaxed, however. If you’re like me, you add significantly more leaf mulch than that to your beds.
Let’s say you have a 4 ft. x 20 ft. garden bed. At the intensity of leaf mulching suggested by the researchers, you would only add 96.8 lbs. of material to the bed (80 ft. x 1.21 lbs/sq. ft.). But I’ve often added many times that much free leaf mulch to my beds at the end of a season, haven’t you? A yard of compost weighs between 1,000 and 1,600 lbs., depending on water content. That means that if I happen to apply what amounts to a yard of highly contaminated (at the worse end of the scale) curbside leaves to my annual beds, I could exceed EPA safety levels for lead in my garden in less than two years. And this assumes that we can trust the EPA guidelines are strict enough, which is a matter of some debate.
What to Do Now?
All of this is incredibly frustrating of course. Human beings somehow seem to turn the most benign sources of natural abundance into new forms of poison. We’re encouraged to buy our way out of the problem, paying exhorbitant amounts for “clean” compost products in 80 lb. bags. What a racket! It’s not unlike what’s happening with our water supplies. We’re forced to either buy bottled water or install an expensive filter if we want to drink clean water.
So let’s not give up on urban leaf compost. We just have to be more strategic in where we obtain it. And also, here’s one caveat: If the leaves will be used in a garden that is strictly ornamental, you’re probably safe to use as much urban leaf material as you like. The lead levels are probably too low to be of concern for pets or casual contact. I would mainly be concerned with gardens used for food production. The details matter. Certain plants, such as berries and fruit trees, for example, are unlikely to draw lead contamination into the fruit. But other plants absorb lead like crazy, especially in their roots. Brassica nigra, or Black Mustard, is a good example. Mustards seem especially good at extracting lead from soils. So are sunflowers.
Where I would take special precautions, however, is with the introduction of urban leaves to annual gardens where cabbages and leafy greens are grown for consumption.
Here are my suggestions for reducing the risk to your health and your soils, while still using urban leaf compost:
- Avoid gleaning from the old City. In Portland, for example, I would not use leaves gathered on the West End or Munjoy Hill, nor in Bayside, for that matter. A lot of the debris from the great fire of 1866, for example is buried in Bayside, and these “historic” areas are often toxic wastelands in terms of the lead contamination of the soils.
- Look for natural lawns. Much as we’d like to see all lawns replaced with permaculture gardens, that’s a long way off. In the meantime, gather leaves from homes with heavy grass cover that doesn’t look like herbicides have been applied. The grass reduces direct contact between leaves and soil.
- Favor new developments. Scout out new developments on the urban fringe, where homes were built after 1978. I suspect someone has a map of Portland with an overlay based on the age of the neighborhood. If so, post it here in the comments please.
Sorry to be the bearer of bad tidings about urban leaves. I wouldn’t take them off your list of permaculture resources. Simply be more selective in where you acquire them. Head for the outer suburbs, and be careful about buying your mulch from organizations that use urban leaf compost as a major component of their mix.
Matt Power is a West End resident and board member of The Resilience Hub (resiliencehub.org), a Portland-based organization that brings permaculture and community together. He is also the author of “The Tiny House Tactical Guide,” and Editor-In-Chief of Green Builder Magazine.