Plant Profile-

Ice-cream Bean

We all love to focus on our edible plant species so we tend to forget that they can only ever be a small component of a much larger system. The simple idea of an edible landscape appeals to human self-interest, until you reflect for a moment how humans are unable to restrain their appetites (much like any other organism). A truly edible landscape wouldn’t be too far from the bedlam when the children were set loose in Willy Wonka’s factory where everything was made of confectionary. A functional landscape that doesn’t just exist for human benefit requires a large helping of supporting species. This predominantly means spontaneous and sincere vegetation that is managed minimally by people, the often overlooked zone inhabited by natives and ferals alike. The intermediate zones strongly feature human managed support species that provide a wide range of functions.

These managed support species need to tick a long list of requirements to do the job well. Most of these demands come from the fairly large scales involved in the intermediate zones coupled with the limited human labour available to manage them. They need to be easy to propagate and establish on a large scale. They need to be fairly fast growing, but well behaved once established. And they need to be compatible with other edible species such as staple field crops and livestock that occupy these intermediate zones at times. One such plant that ticks all these boxes is the ice-cream bean, Inga edulis.

The tree grows up to 30 m and is quite fast growing. The large leaves are almost blackish green and between the canopy shade and leaf litter effectively suppress virtually all weeds beneath them. The wood is fairly soft and easy to cut, and the trees recover very strongly from pruning at a young age. The firewood is adequate, a bit spongy and average heat content. It rots too rapidly for construction. They produce white flowers in late spring that turn into 15 cm square pods that contain firm green seeds surrounded by a sugary white pulp. This is where the ice-cream part of the name comes from, though I find the fruit rather revolting (wet clown hair was my description). It deserves a place on the long list of “rare” fruit that “go well with actual ice-cream”, just like shredded cardboard is palatable with a big scoop of vanilla on top. The genus is quite large with 300 species, among which are some with much tastier fruit pulp that is supposedly more deserving of the common name. The seeds themselves are also edible after boiling and roasting/frying, apparently making a common snack in Central America. I haven’t tried this for myself yet but plan to give it a go this season, so do your own homework before trying them.

The seeds themselves have the interesting trait of being “recalcitrant”, meaning that they are unable to go dormant or dry out and must grow immediately. This is a more common trait in tropical species such as jackfruit and mango. When they come out of the fruit they are basically a naked green embryo with no seed shell to protect them. The seed can be extracted from the surrounding pulp easily by soaking them in a few changes of water. This causes the pulp to swell up and pop off easily. Seeds can be sowed individually in small tree tubes since they readily give close to 100% germination. A 5 cm square tube sowed in late summer will grow to a 20 cm seedling by the following spring. They can be planted out as soon as the summer monsoonal rains arrive. The seed also grow readily by direct sowing, but do much better if they have no grass competition and some shade, though I have had seedlings emerge from thick ungrazed paddocks a few years after sowing.

Where Inga edulis really stands out is as a support species for field crops. The Inga Foundation (http://www.ingafoundation.org/) has developed a system to replace damaging slash and burn agriculture in the tropics by encouraging farmers to plant carefully spaced rows of Inga. With the correct density of planting the young trees form a weed suppressing canopy after a few years. They can then be cut and the biomass used to create a thick mulch. Conventional crops can be planted into this space. Inga is a nitrogen fixing legume and also accumulates phosphorus and other minerals from the subsoil. These nutrients become available during the cut back cycle. The Inga regrowth is managed lightly as the crop grows and is allowed to close back over afterwards. Using this system land that would previously be burned then abandoned is kept productive while preventing erosion and reducing damage to the wider forest. Shade tolerant species like turmeric can be grown as an understory between heavy cuttings if the canopy is managed.

After learning about this system I decided to try it on my creek flats. This space contains the best silty soil on my farm, flat enough to tolerate digging root crops, divided into three approximately one acre parcels that are between 200 m and 400 m from the house. This is the best space for field crops, including staple tubers, since they don’t need frequent maintenance. I planted five test rows of Inga seedlings, varying the spacing between rows from 3.5 to 4.5 m apart to test the effect of variations on the recommended 4 m spacing, plus a double row 8 m apart. I figured I could interplant another row there at 4 m spacing later if I wished. The trees were about 1 m apart within each row. Around 30% of the trees needed replanting with seedlings once or twice and a few gaps still remain but have been direct sowed now. The planting was later expanded south and across a small gully with more seedlings. Some seedlings were planted under pigeon pea “nurse shrubs” but this was generally a negative influence as these Inga failed to thrive until the pigeon peas were cut back and killed.

After five years the largest trees have reached about 8 m tall with trunks about 10 cm across at chest height, though most are closer to 5 m tall and 5 cm diameter. Where there were no large gaps in the canopy the model worked as advertised, with no significant weed growth. Those weeds present were etiolated and weak, needing only a gentle tug to remove. In the traditional system the leafy canopy is stripped to mulch the entire growing area. I decided to instead pile up the trimmings to grow cucurbit vines over. This leaves more open spaces in between to grow amaranth (that can’t germinate through mulch) and maize. I managed to cut back about half the area (about a quarter of an acre) over the space of about 8 hours spread over a few days. Working mostly under the shady canopy made the job quite pleasant, and the physical effort of sawing and dragging small branches was much nicer than hoeing and digging out weeds and grass.

Some of the trees were cut down earlier in winter as a trial and have since grown back nicely, with numerous thinner shoots. This spot has more weed regrowth so might be cleared and planted later if time allows. This regrowth should make perfect goat fodder since it is mostly leaf with little wood, making a bundle quick to cut and efficient to carry back to the house. The goats adore this plant though rapidly ringbark any trees they encounter. Faced with a forest of small trees they may focus on the leaves first before turning to the bark, so it may be possible to incorporate direct browsing in the future when I am preparing a new cropping space. Inga also nicely fit within the annual rhythms of the farm, benefitting most from cutting back in late spring just as they flower to prevent fruit forming. This may be necessary since Inga can form carpets of seedlings in some localities, causing it to become a weed concern, especially on red soil. In my more clay soil I only spotted a few self-sown seedlings growing where they shouldn’t be, and they came out cleanly with a weak tug. The soil itself under the Inga was of excellent texture, spongy underfoot and should make an ideal growing space (far better texture than the soil in my zone 1 vegetable garden). This cut back timing aligns nicely with the bulk of my staple crops that are planted as soon as the summer rains arrive, with a potential follow up crop in autumn-winter. I will put up future articles on these staple crops as they grow under the Inga alleys through the season ahead.