Carbon “Fixing” Plants

Most gardeners are aware of “nitrogen fixing” plants; those plants, typically legumes, that have formed symbiotic relationships with soil bacteria to take airborne nitrogen and fix it into water-soluble form.  As far back as the Romans, humans have fostered this natural occurrence to increase yields and increase fertility.  What we hear about much less is carbon “fixing” plants – in Permaculture speak – biomass or mulching plants.  In the past year, as I have learned more and more about soil ecosystems, I have become far more convinced that these “carbon fixers” are even more important in early successions as we heal our suburban ecosystems.

I have developed a mantra over the summer as I have worked to sloganize what it is that we are doing.  The result has been my oft-repeated imperative to: Heal the Soil, Store the Water, & Plant Useful Plants.  In a nutshell it is a roadmap to maximize the potential of any garden space – if the soil is alive it has more nutrients cycling in it, if the soil has sufficient water stored under mulches and in humus then  the potential increases again.  And once these two are in place, one can maximize the solar potential by filling in the canopy.  Of these, I feel the first is the most important as it makes the others possible.  And to heal the soil, you need carbon (organic matter).

Plants are truly the conduits of energy into the earth – by capturing solar energy and turning it into simple sugar they supply the foundation for 99.99% of all life on the planet, whether it is through root exudates in the rhizosphere or nectar through their flowers, plants make life possible.  But life also needs carbon, and most life on the planet gets its carbon from plants in the form of organic matter – the main food source of the bacteria and fungus in the soil.  But some plants do this better than others.

There are the grasses with their dense root systems that build soil visibly every year, and there are trees that cover the ground in a thick blanket of leaves every year.  Finally there are other plants that form stalks and stems that form significant amounts of soil as well – the tall grass prairies, corn/sunflower stalks, and windblown branches from softwoods such as willows, sycamores, and poplar.  What these have in common is tough, long carbon chains in their cell structure that resists decomposition.  This resistance to decomposition -in lignin, cellulose, etc-  is what forms humus.  And it is humus that forms rich soils.   This is something to be very mindful of as we mulch our gardens.

If you were to build a compost pile of nothing but greens, with only the barest amounts of “browns”, the pile would heat up very quickly, but would decompose down to almost nothing – 75-90% of the bulk would be gone.  That is because greens lack cellulose and lignin and are mostly water and nutrients – vital to soil life, but almost completely consumed in the decomposition process.  Compare this to a similar sized pile of shredded leaves.  It will take 4x as long to decompose, but the result will be 400% more humus with only a 25% reduction in size.

In most of my permaculture guilds I have stressed the green mulches of comfrey, sorrels, chives, etc to pull and cycle nutrients, and for several years I have imported my cellulose and lignin in the form of dozens of yards of wood chips.  Now that my gardens are maturing, I am starting to pay more attention to including plants specifically to “fix” carbon to maintain soil humus levels.  In the 6 years I have lived here I have lost about 1-2″ of total height in our 3 oldest perennial beds compared to the sod.  That is because the humus in the perennial beds is degrading over time, while the roots of the fescues in the lawn are forming .25″ of humus a year.  I had not been mulching these beds much as they have almost completely closed “canopies” of thyme ground cover that I didn’t want to smother.  I will try mulching more aggressively this year.

While all my guilds include nitrogen fixers such as leadplant, false indigos, New Jersey Tea, Serviceberry, etc and green “mulch” plants such as Russian Comfrey, I am beginning to either add in “carbon fixers” and am planting guilds specifically to grow brown mulches.  Some of this I am accomplishing by letting box elder seedling mature in my guilds as they sprout in the wood chip mulch.  In one spot, I have planted a 20 tree “short rotation coppice guild” of willows, poplar, and box elders.  In my annual gardens I am taking much inspiration from John Jeavons and planting “stalky” plants such as corn, sorghum, and sunflowers specifically to produce compost carbon in addition to edibles.  Once you change your mindset it isn’t overly hard.  Of course, having a chipper to use can become important as well – though many plants such as sunchokes, and weeping willows produce stalks and “leaf” mulches that are laid down without chipping and fast growing trees such as Empress and Sycamore have HUGE leaves that really add up.

As my gardens have matured, and I seek to support the soils I have built using primarily the inputs of my site, I find myself planting more and more of these carbon trees to grow my soil.  In doing so I help to maximize the potential of my site by using these fast growing plants to sequester carbon to build the soils beneath my edibles, while also pulling carbon from the air to heal our climate.

Prime Short Rotation Coppice Trees:

Willow, Hybrid Poplar, Black Locust, Box Elder, Empress, Sycamore, Ash, Hazelnut, most Standard fruit trees (annually pruned)

Prime “Carbon Fixing” Annuals/perennials:

Sunflower, Corn, Sorghum, Cupplant, Sunchoke, Small Grains, Amaranth, Quinoa, etc.

Many of these trees are useful in many other regards – Black Locust is a nitrogen fixer, provides rot resistant wood, and its flowers are a great early nectar source.  Black Maul willow is one of the most striking plants I have ever seen and the new growth makes incredible baskets.  Hazelnuts and Sunflowers are some of the best ways to grow healthy fats in the northern hemisphere.  Carbon gardening is by no means boring or a wasted effort!

Productive soils need to have all their nutrients cycled, not only the water soluble ones, but also carbon to replenish that which is lost due during the respiration of the soil organisms.  If we are to truly garden in the spirit of nature, we need to make sure that the carbon is replaced with the same diligence that we give nitrogen, phosphorus, potassium, calcium, and the 96 other macro and micro nutrients.



13 Responses

  1. BBC’s recent article on Ascension island (link above) reminded me of your efforts there. It seems very few species had colonized it naturally, so the British empire put in a mix of trees for humus, and useful tropical plants. Somehow, a healthy ecosystem formed.

    • That was a fascinating article. What I took away from it is the need to rethink the mania around “invasives” and persistent weeds. We have millions of acres of farmland that has been denuded to 1-2% organic matter by industrial agriculture. Learning what plants will grow on these wastelands will be very important as we look to reconstruct the soil ecosystems.

  2. Do you have any information for which plants to grow when your soil is contaminated with Cadmium and Lead? Thanks for any information on this subject!

    • I do not – I would recommend talking with a soil remediation specialist on that as heavy metals can be a significant health risk. Soil with high humus content can “lock up” some substances, but again – contact an expert.

    • You might check out the Worcester roots project. They seem to recommend lemon scented geranium, which builds up high levels of lead and/or cadmium in its tissues if they’re present in the soil, and can be safely landfilled.

      I’d also recommend a no-till method of cultivation, to avoid bringing any lead up to the surface, or kicking up any breathable dust.

      Lastly, I’ve read that cations like lead and cadmium are more soluble in acidic soil, so it might make sense to mulch the area with your neighbors’ Christmas trees while the geranium plants are doing their phytoremediation thing, then once the soil tests mostly clean, add enough agricultural lime to make the soil slightly alkaline.

      • Thanks Joel, I will check that out! (and do you mean the real Geranium, or the Pelargonium – which so many people name Geranium)

      • You’re right, it isn’t true geranium. I looked it up, and the species is Pelargonium Frensham. They also have articles on soil amendments at their website.

      • Thanks Joel for the link to the Worcester roots project, although they don’t provide easy information on what plants to grow – I only could find the Pelargonium that you mentioned.

        I’m a no dig person, so no problem there!

        How do yoy know the neighbors have huge Christmas trees??? I’m wondering where you read this stuff?

      • I had no idea re: the size of any particular neighbor’s Christmas tree, but heavy lead contamination plus scarce enough land to warrant bioremediation suggested an urban environment, so I inferred that you might have enough neighbors within scrounging distance that January would be an excellent time to collect conifer mulch.

        >I’m wondering where you read this stuff?

        I just read Toolbox for Sustainable City Living, by Stacy Pettigrew and Scott Kellogg. I also participate in the forums at

        By the way, that latter source recommends using loppers to turn whole trees into mulch, rather than a chipper: not only does it save the expense of fuel & capital equipment, but a branch is easier to place, and offers better coverage per mass, than a heap.

  3. Thanks Joel for all your advice and links!

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