Arrowroot vs Comfrey Trial – Which Is Better For Growing Biomass In A Warm Climate?

“I live in Townsville and wanted to know how your trial of arrowroot vs comfrey went, so I can learn which produced the most biomass for soil improvement”, asks John.

Reply: I found that Queensland arrowroot (Canna edulis) requires 1/5th of the water to produce twice the biomass of comfrey (Symphytum x uplandicum).

At the outset, I already knew how incredibly productive and versatile arrowroot is. People who followed my garden’s development are familiar with my early reliance on arrowroot to mass produce leafy ‘cut and come again’ material for compost-making and mulching from 2003 to 2007.

During the Millennium Drought, arrowroot required less than 1 litre of water per square metre of soil per day for continual production.

I recorded a story for Gardening Australia showing how I use arrowroot to:

  • Make mulch;
  • Make compost;

and how to:

  • Select and prepare the rhizomes as food;
  • Use the leaves as disposable plates;
  • Use arrowroot to provide shelter in a food garden;

I also make arrowroot starch from the rhizomes.

I get around 45kg rhizomes from 10 sq metres of soil. When the outer skin of new arrowroot rhizomes is shiny and purple, they are ready for harvest.

a cultivated garden bed with comfrey and arrowroot planted in blocks
The trial begins, November 2015

An organic farmer I know in the Gympie region (Queensland) plants rows of arrowroot, allows them to establish, and then lets his pigs eat them. They dig up the soil as they grub for and eat the rhizomes, so when he takes them out it is ready dug and fertilised for follow on crops. This is a form of crash grazing (personal conversation).

All you can do with comfrey is make weed tea or compost. Yes, the flowers do attract bees, so do flowering crops, but to allow confer to flower means you forfeit cutting them back, which means reducing biomass.

What Australian gardeners overlook is that comfrey is a cool temperate perennial, unlike arrowroot which is a warm climate perennial. Very few places in Australia have a moist, cool temperate climate resembling Northern Europe.

So, in the subtropics, the volume of water comfrey requires to keep hydrated, let alone yield biomass, is astounding. In this trial, I waited until the comfrey reached wilting point (pictured) before watering the trial bed, but otherwise I relied on rainfall to water the plants.

For some, saving water in drought, or saving water during Queensland’s dry spring, isn’t something they factor in or value.

I watered when the comfrey reached wilting point

I conserve water. I live in Brisbane which has a cooler subtropical climate than Townsville. I imagine you could easily double the amount of water required to grow comfrey that far north, plus comfrey is very likely to rot in your hot, humid, wet season. Unlike arrowroot, which copes better with these environmental stresses.

The trial results

After three cuts from January to May, in a 10 square metre garden bed set out as 50:50 comfrey: arrowroot were:

  • Comfrey yielded 11.64 kg biomass;
  • Arrowroot yielded 21.34 kg biomass;

and

  • Arrowroot required 1/5th of the water to thrive!

John, You have a good memory – ABC TV filmed the set up of the trial in November 2015. They didn’t return to film the results, but I keep records.

Kind regards,

Jerry Coleby-Williams
Director, Seed Savers’ Network
11th May 2022




2 Comments Add yours

  1. Nancy Siow says:

    With the increasing demise of nurseries throughout Brisbane and therefore depletion of easy source of supply, where can I easily get arrowroot or comfrey plants and other useful plants and without paying a premium for them?

  2. Patty Munro says:

    Thanks, Jerry. Very interesting.

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