What to do after planting your green beans

Although beans are generally a quick and easy crop to grow, problems can occur before the plant emerges.

Modern bean varieties are generally bred for uniform maturity to save labour and mechanical harvesting.
Photo: Bill Kerr

Management must be adapted to soil type and condition and the likelihood of heavy rain. The latter can cause crusting in some soils, especially those that have been overworked and have a low organic content. I have come across soils where a heavy downpour occurred after planting. When the sun emerged, the surface was so hard that the beans snapped at their necks attempting to penetrate the crust.

Another problem with crusting soils is that the soil below is typically very wet. The crust seals the surface, reducing available oxygen and creating conditions for the development of soil-borne diseases. When crusting occurs at an early stage of germination, some farmers run a rolling cultivator (millipede) through the land to crack the crust. This will be successful only if the beans are planted deep enough to avoid being damaged in the process.

Another alternative is to irrigate lightly to soften the crust so that the beans can emerge easily. Do not do this too early, though, as the environment under the crust will become even wetter, and the crust will again harden after a day of sunlight. Get the timing right by scratching daily in the row to determine when the beans are about to push through the crust.

Cutworm can be combated by applying a suitable insecticide with a pre-emergence herbicide. I have often come across farmers who took a chance and did not apply insecticide, and reaped regret instead of beans.

Nitrogen management
Although beans are a legume and can have nodules on the roots to utilise nitrogen from the air, they are not the most efficient producers of nitrogen. What’s more, the process kicks in a bit late for the required initial boost. For this reason, beans for the fresh market are usually not even inoculated. Very often, especially in healthy soil, the microbes that initiate root nodulation are present in the soil anyway.

As you need a fast, uniform getaway, ensure that the crop has sufficient nitrogen by adding (limestone ammonium nitrate) LAN a week after emergence. This need only be a light dose of about 100kg/ ha, band-placed 5cm to 8cm from the plants. In some cases this may be unnecessary, but it’s not worth taking a chance.

Start harvesting about two months from planting, and be careful when applying nitrogen at this stage. If too much is available to the plant at the time of flowering, yield will be reduced. This applies to many crops where the fruit is harvested and not the vegetative component. You can see the effects more clearly in seed production; plants that have a healthy medium colour produce many more pods than those with dark, lush foliage.

Not all the flowers on the plant will necessarily develop into marketable pods. The number will depend on the conditions. But the plant does need a full canopy of leaves to capture the sunlight falling on the area. If, after the initial LAN application, the leaves become too light and growth is stunted, apply a further light dose of LAN. When the crop is well set, apply a further dressing if you want the pods to become slightly darker.

I once visited a farmer who was being advised on his fertiliser programme by a consultant. My initial impression was that gem squashes, not beans, were growing on the land due to the very large, dark leaves caused by excessive nitrogen.
The farmer asked me how it was that his neighbour, who neglected his crop, still managed to get 12t/ha, while he struggled to get 8t/ha.
The lesson? Nitrogen management is very important with this crop.