Soybeans – Ever so Short

The ILSoyAdvisor received a question from central Pennsylvania last week:

“My question is that my growers are seeing flowers on plants that are almost 1.5-foot-tall, which is much smaller compared to what we saw last year (or in normal years). I am wondering what happens to plant growth after the plants begin to flower. In other words, do plants keep growing and keep producing more nodes after they start flowering? My understanding was that once the plants start flowering, they don’t grow much in height and or nodes. Is that true?”

This is a very good question and 2019 typifies a year of short plants associated with late planting followed by quicker initiation of flowering.

Soybeans grown in Illinois are indeterminate and continue vegetative growth after flowering starts. Soybeans begin flowering after accumulating a couple nodes. However, it really commences at Summer Solstice as night length increases. Soybeans will continue to flower all through July and into early August at the top of the plant.

This season soybeans were planted mainly in June and commenced flowering. Embryonic node structures are formed early in development, but laid down later as the plant grows. There was so little time between planting/emergence and flowering that fewer nodes and internodes formed, resulting in shorter plants. Hopefully, branching and lower pod set will compensate for loss of nodes and height.

Height does not always correlate with yield. I cite the 2012 drought year. Soybeans stayed short and growth was relatively dormant all summer, but the crop survived. At the same time corn was extremely stressed and, in some cases, literally died or yields were extremely low. However, when the rains returned in mid-August soybean plants renewed growth, pods begin to fill and yields were often just below average. This taught us that soybeans can survive a drought and that short plants don’t necessarily equate to reduced yield.

When planting soybeans is delayed, vegetative growth is reduced since flowering starts as soon as the plants have one or two nodes. However, the plant is indeterminate and will continue vegetative growth producing more nodes and internodes, but due to the lateness of planting and so little time before flowering, fewer nodal points are laid down on the main stem. Late-planted soybeans will be shorter with few nodes, but with more pods occurring lower in the canopy as more sunlight penetrates deeper through the canopy.

The one plus that comes with late planting and shorter plants is that a shorter crop won’t lodge, which would lead to some harvest losses.