Sweet potato and katuk (sweet leaf/sayur manis)

We have been playing around with companion planting at Project Canossian. Actually it’s more of stacking plants on top of one another rather than just adjacent. Lots of failures with some successes. It is something we have been experimenting with since our days at Project Green Valley. There are many good reasons why you would want to companion plant by vertical stacking:

• Better use of sunshine: Here in Singapore 1 degree north of the equator, the sun is overhead throughout the year. As Bill Mollison says, a resource that isn’t properly utilized can become a pollution. Sunshine is photosynthesized by leaves and is turned into a yield (for us or other living things). However, when this strong sunshine falls on the soil many things happen. Soil water evaporates, soil organisms (e.g. earthworms) die from the heat, ambient temperature increases due to radiation. A single layer planting of small vegetables or herbs does not use the sunshine efficiently. Lets talk about planting lettuce as an example: for most of the growing duration, most of the sunshine will fall between the tiny seedlings onto the soil. Even when the lettuces are fully grown and touching one another, the leaves are not able to utilize all that good sunshine. One layer of leaf can only photosynthesize so much sunlight, after which the remaining passes through the leaf. Which is why the best use of sunshine here in our climate is still a forest. Almost all of the solar energy would be extracted as the sunshine passes through the many layers. Nothing much is left by the time it reaches the forest floor.
• Better use of rainfall: Likewise, we are blessed (or, to poly-tunnel farmers, cursed) with heavy rains throughout the year. The kinetic energy of falling raindrops causes soil compaction and sheet erosion when not absorbed by foliage or mulch. Having more leaves to intercept and direct the water flow down the stems allows the soil to drink more gently. If there are not enough roots in the soil to absorb the rain it flows away, causing leaching of nutrients.
• Shading out weeds: Nature doesn’t waste. Solar energy that falls on soil will be taken up by weeds. Mulching helps but we have seen weeds here that can penetrate mulch (cardboard or leaves) just to reach for sunlight.

For all its benefits, why do so few farmers stack their planting vertically?

One reason might be that vertical stacking looks too messy to most gardeners. They tend to idealize neat straight rows with equally spaced veggies of uniform height. To which I quote Bill Mollison:

Order is found in things working beneficially together. It is not the forced condition of neatness, tidiness, and straightness all of which are, in design or energy terms, disordered. True order may lie in apparent confusion… Thus the seemingly-wild and naturally-functioning garden of a New Guinea villager is beautifully ordered and in harmony, while the clipped lawns and pruned roses of the psuedo-aristocrat are nature in wild disarray.

Another reason might be because the yield of one product might decrease. However, total yield might be higher. For example, planting A or B by itself can give you 10kg per bed, but planting A under B will give you 6kg of A and 8kg of B. This might be an issue if you need to meet a minimum bulk for logistical or marketing purposes. Harvesting might also be more difficult. This I have to admit, pulling out a tapioca tuber without damaging or uprooting an undergrowth plant can be challenging!

Lastly, vertical stacking can be difficult to implement on an industrial scale. The gardener needs to pay close attention and observe the ever changing state of the garden. You have to play with time niches, which means you are not just stacking in space but also stacking in time. For example, an undergrowth veggie under a papaya tree might not work when the papaya tree is young, but would work when the tree grows taller and lets in more light from the side. Edges and differing micro-climates are advantageous, both of which are lacking in industrial-scale monoculture farms.

Considerations for Vertical Stacking

• Spatial needs: You need to know what space each plant will take up. Some sprawl on the ground, some are vertical reaching shrubs, some are climbers. Putting two of the same kind together might create competition.
• Sunlight needs: Know how much sun each plant needs. Some plants are tolerant of a wide range of sunlight and their flexibility is an asset.
• Growth speed: Certain plants are much more vigorous and fast-growing than others. Understand how fast each one grows. For example, planting sweet potato too early with papaya might smother the papaya seedlings.
• Micro-climate: Observe your surroundings to see how they affect your planting. For example, the sun shifts throughout the year and light enters from different angles, which means certain combinations might work only during certain parts of the year. A big white concrete wall nearby will reflect sunlight and increase temperatures. A shady tree overhead means more moisture and lower temperatures.

At the end of the day, it’s all about experimenting and that’s where the fun is! After all that has been said, here are some simple combinations that are working well for us:

Katuk + sweet potato: The vertical growing katuk (sayur manis) with sweet potato on sides. In this case, sweet potato is grown more for leaves as vegetable since tubers would be hard to harvest without damaging katuk. The tree in the background is actually stacked on top of katuk, which likes cool and moist.

Tapioca + butterfly pea: Tapioca works well as a trellis once it grows tall and the understory is more bare. Butterfly pea has edible flowers used for natural food coloring and it also fixes nitrogen for the tapioca.

Tapioca + butterfly pea: We tried sweet potato but they didn’t thrive in the partial shade under the tapioca.

Tapioca + asystasia: Tapioca helping asystasia (Ganges Primrose) with some shade while asystasia smothers grasses.

Leucaena + chives: Leucaena (Petai Jawa) fixes nitrogen and provides shade through thin foliage for chives.

Papaya + katuk: Only suitable when papaya has grown taller. Papaya protects shade-loving katuk.

Papaya + katuk: Papayas have grown taller. The katuks did not like the sun and heat when we first transplanted them but are growing back better now.

Papaya + sweet potato: Might reduce yield of sweet potato and make harvesting tubers more difficult.

Papaya + agati: Three papayas with an agati (Sesbania grandiflora/hummingbird tree) in the centre that fixes nitrogen and produces edible leaves and flowers.

Banana + kang kung: Bananas just transplanted and should not shade out kang kung as long as the clump does not grow too big.