Plant Profile- Sword Beans (The Beanogenesis Chronicles)

Staple legumes appear in the majority of agricultural systems. They produce only modest amounts of carbohydrate (a role dominated by grains or tubers that give higher total calorie yields). Their most valuable role is as a form of storable protein (to complement variable flows of animal protein). Protein from legume seeds is usually inferior to animal sources, due to lower bioavailability and the presence of toxins, but if the alternative is protein deficiency then a sack of dry beans can be a lifesaver.

Under my peculiar conditions I have trialled just about every staple legume species imaginable. The main barriers are my weird low calcium/high magnesium soil, abundant weed growth whenever we aren’t in a drought, and considerable pressure from our local pod sucking bug. This pest inserts its proboscis into developing legume pods, damaging the developing seeds. The only legume which had shown some promise was lima bean, due to its habit of cropping through the end of autumn when pressure from the bug was lower.

I acquired my first swordbeans pretty much by accident, initially pegging them as some barely edible permaculture weed that nobody knew how to eat. The first species I grew was C. gladiata. The enormous, bright pink seeds germinated readily into robust seedlings that ignored my trellises and scaled the hedges surrounding my vegetable gardens. I sowed them in an area going back into fallow, so the plants got zero follow up attention. The next winter the giant, thick walled pods were scattered around the hedges, providing a modest crop. I have researched the required steps to detoxify the seeds but haven’t scaled up enough to try cooking them myself. The consensus for the cultivated species seems to be soaking, then cooking in 2-3 changes of water (and most critically cycling the ingredient in and out of your diet seasonally). For a long time I assumed lima beans would be the better alternative, and intended to begin hand crossing the half dozen strains I had gathered in coming years.

My outlook on Canavalia changed when two other species appeared on my radar. C. ensiformis is the other main cultivated species, with smaller white seeds. C. papuana is a wild native species from northern Australia and Papua New Guinea. Its undomesticated nature is evident in its thick seed coat which needs abrasion or hot water treatment to break its dormancy. This species is also likely to have more toxic seeds. Given the robust performance of C. gladiata I decided to plant all three species side by side and do deliberate hand crossing. The aim was a locally adapted staple legume that I could plant all through my overgrown orchard and hedges, needing nothing more than sowing and harvest (and careful cooking).

Once the crops were started the first barrier to overcome was simultaneous flowering. They were all sowed at the same time, but there was no way to know in advance how each species times its reproduction. Luckily this wasn’t an issue and they all flowered together in February (though C. gladiata was the earliest, and C. papuana latest).

The next barrier was how to arrange cross pollination. Most legumes (especially domesticated ones) are predominantly self-pollinating. This is probably an adaptation from when the original wild species were moved into regions where they lack suitable pollinators. Interestingly I observed a species of native solitary bee enthusiastically working the flowers, but I couldn’t rely on them to do outcrossing for me. I scrounged around for a piece of gauzy cloth, cut it into handkerchief sized squares, and started wrapping spikes of flowers that were at a convenient height on the trellis (held in place with a wooden clothes peg).

Using a set of tweezers, I applied the technique used for hand pollinating sweet peas. Anthers start to develop in the late afternoon in unopened flowers. The petals are squeezed open, often using the tweezers to split them apart. Then a gentle downward motion will knock off the ends of the anthers while usually leaving the female stigma intact. I usually had a cloud of tiny mosquitos around my ankles at the time, so sometimes I hurried and damaged the flower. The next morning I had to get up just as the dew was rising to beat the solitary bees to the intact flowers. I found the easiest method was to collect whole flowers from one species in my pocket, then peel back the petals off the anthers. With my other hand I could squeeze open the anther free flower of a different species from the day before, then dab the anthers from one species onto the stigma of another. I would then reapply the square of mesh to prevent bees from self-pollinating each species.

I didn’t bother labelling individual crosses, but selected a plant of each species to receive pollen from either of the two other species. This ensued that all six possible crosses were conducted. In the end I think I got around a 10-20% rate of pod setting, with no indication that any crosses were more or less incompatible. All going well I should end up with a few dozen seeds to grow out next season. I plan to plant about half of them (around one row) alongside another row of pure C.  gladiata.

The ultimate aim is to identify the most promising hybrids to backcross to gladiata since it seems to be the closest to my ideal plant. C. ensiformis is a bit too domesticated for my tastes- the plants set most of their pods low, dragging in the dirt, and the vines are almost bush shaped. By contrast C. papuana is too wild, with small, toxic, hard shelled seeds, pods that shatter. A complex hybrid of all three, selected back toward the best traits of C. gladiata will most likely produce a lineage which can provide large quantities of seed in return for little more effort than sowing and harvesting.

I still plan to breed my lima beans, but see them as being a crop suited to areas with more intensive management (namely vegetable gardens and silty lowlands in my icecream bean alleys). Sword beans will fill a complementary row in less managed spaces, where their hardiness allows them to produce a useful yield with minimal involvement on my part.

Our friendly pod sucking bugs that ruin many legume crops

C. papuana getting attention from a local solitary bee species.

The larger flowers of C. gladiata. The one on the lower left had its anthers stripped.

(Hopefully) hybrid pods on the refined C. ensiformis.

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