Soybean Growth Stages

If you live anywhere near a cornfield, you may have noticed that every other year, the corn stalks are replaced by something much shorter. More often than not, this is a crop of soybeans (Glycine max)!

Farmers do this because soybeans are legumes, and legumes have a sort of plant superpower. Legume roots work with special microbes to take nitrogen particles from the atmosphere and ‘fix’ them into the soil. Nitrogen is the most essential nutrient for plant growth, so this process directly benefits crops (like more corn) planted after the soybeans.

Of course, there’s more to soybeans than just what they can do for other plants. In this article, I’ll teach you a bit about soybean growth stages and offer other reasons to try growing them at home!

Conditions for Growing Soybeans

According to the University of Nebraska, the earliest domesticated soybeans were likely grown in China circa 11th century BCE. However, about 80% of soybeans come from North and South America.

Though most soybeans are grown in large agricultural fields, nothing stops home gardeners from adding this nutritious crop to their rotations. (Edamame is just one popular dish made from young soybean pods!)

Just like most other legume varieties including green beans, soybeans are a warm-season crop that prefers daily temperatures between 68 and 86°F. For home gardeners, I recommend starting soybeans in the spring when soil temperatures are at least 60°F. They will grow in USDA Zones 2 through 11. Some varieties are bred to tolerate hotter or cooler temperatures, so keep this in mind when selecting plants for your garden.

Select a site that receives at least 8 hours of sun per day. Too little light will decrease the total yield of your soybeans.

Soybeans need plenty of water to offset the heat of summer. Established plants require an average of 1 inch of water per week. 

Commercial farmers space soybeans closely together to prevent weed growth and maximize efficiency. Keep in mind, however, that tight spacing can impede airflow and increase the chances of your soybeans suffering from a fungal disease like white mould.

You can also grow soybeans in containers! This option is great for a patio garden or any similar small growing space. Use raised beds or individual containers (at least 12 inches deep) for the best results.

Soybean Growth Rate

The average growth rate of soybeans varies throughout the plant’s life cycle. Here are some good, basic guidelines to keep in mind:

  • Soybeans should reach 6 to 8 inches tall within 30 days of seedling emergence
  • Soybeans should reach 12 to 14 inches tall within 50 days of seedling emergence
  • Soybeans should ideally be at least 17 to 22 inches tall before flowering

Soybeans typically grow up to 3 feet tall in cultivation, though some varieties can grow up to 5 feet tall. Note that environmental factors (e.g., drought) can also impact growth rate and mature size.

Growth Stages of Soybeans

Soybeans require 100 to 150 days to go from seed to mature pods. Remember that soybeans used for edamame are not fully developed and are usually harvested after just 70 to 85 days.

All soybean plants can be classified by their growth habits. There are two distinct patterns that soybeans may follow (you may recognize these terms if you’ve ever grown tomatoes):

  • Determinate soybeans grow to a specific height early in the season and then begin flowering. Once flowering occurs, the plants will not produce any bigger.
  • Indeterminate soybeans continue growing even after flowering. The flowers toward the bottom of the plant bloom first, while new growth forms at the tips of the stems. New growth can continue for several weeks as flowering moves up the plant.

According to North Carolina State University, determinate types are more common in warm climates and vice-versa. The only thing you need to remember is that the vegetative and flowering stages overlap quite a bit in indeterminate types.

1. Seed Germination

Soybeans are one of many plants whose seeds we commonly eat. Each pod contains three soybeans. Each soybean is a seed that can be grown into a new plant. 

Soybeans germinate incredibly quickly and easily. Germination often starts just two days after planting!

Newly planted seeds can survive temperatures as low as 35 to 40°F. However, the ideal soil temperature for germination is 77°F. Soil temperatures below 60°F will slow down germination considerably.

The first step in germination is water absorption, also known as imbibing, when the soybean takes in moisture from the soil through its seed coat. This process takes less than 24 hours.

Once the seed absorbs enough water, cells inside the source begin dividing, and the embryonic plant grows. The main parts of the plant at this stage are the radicle, hypocotyl, and cotyledons.

The radicle is the first part to emerge from the actual seed. This is the plant’s first root. It grows into the soil to stabilize the source and take up moisture and nutrients.

The hypocotyl and cotyledons follow shortly after the radicle. These structures represent the plant’s first stem and leaves and will naturally grow toward the soil’s surface.

2. Seedling Emergence

Soybean seedling emergence occurs typically within 5 to 10 days of planting. Emergence occurs slightly sooner when seeds are planted near the surface and/or in loose soil.

Your seedling may not look like a soybean plant when it first emerges. This is because the cotyledons are a different shape from the plant’s mature foliage. Cotyledons are small, thick, and round. They provide energy during the 7 to 10 days before the soybean develops ‘true’ leaves.

The next two leaves of the seedling are also unique. These are unifoliated (single) leaves. Each soybean seedling develops two opposite unifoliate leaves directly above the cotyledons. At this point, the cotyledons may begin to die off.

3. Vegetative Growth

All leaves that develop after the unifoliate leaves are trifoliate. These compound leaves consist of three leaflets — similar to a clover leaf. Trifoliate leaves are common in nearly all legumes.

Vegetative Growth

New trifoliate leaves will continue emerging from the top of the stem (the shoot apex) every 3 to 10 days. As the soybean plant matures, growth will naturally speed up.

4. Flowering

The next natural phase in the soybean life cycle is flowering. Soybean flowers form at the branching points along the length of the stems. Most varieties boast purple or pink flowers. 

It takes 3 to 10 days for the soybean plant to reach full bloom after the first flowers appear. Indeterminate varieties can flower continuously for 14 days or longer.

Flowering

Flowering is not determined by the age or size of a soybean plant but by day length. This mechanism (which is common in a large number of plant species) is called photoperiodism.

Soybean flowering is triggered by shortening days and lengthening nights. In many areas where soybeans are grown, this conveniently coincides with the summer solstice, so it is easy to track.

(I must be honest and say that the above explanation has been simplified for brevity and clarity. In practice, different varieties of soybeans respond to day length in slightly different ways. The date when soybeans are planted or geographical location can also influence when flowering is triggered.)

5. Pollination

Soybean flowers are ‘perfect’ — each flower contains functioning male and female reproductive parts. Soybeans are also self-fertile, so the flowers on a single plant can pollinate themselves. This contrasts plants that require cross-pollination from a second plant of the same species.

In the average soybean field, most pollination occurs when the anthers of a flower incidentally brush against the stigma. No outside pollination is necessary. 

While bees and other insect pollinators aren’t required to produce a soybean harvest, their presence can greatly improve overall yield. There’s also evidence that soybean nectar is a great food source for local pollinating species.

6. Pod Formation

Pods typically from 5 to 21 days after flowering begins. Note that growth is tracked by comparing the progress at the top of the plant — i.e., you may see small pods forming while there are still actively blooming flowers on the same plant.

The first benchmark in pod formation is a pod that is at least 3/16 inches long and located at one of the plant’s four uppermost nodes. This phase is known as the ‘beginning pod’ and, on average, occurs about three days after flowering.

The second benchmark is when one of those uppermost pods grows to 3/4 inches long. This growth phase is known as the ‘full pod’.

7. Seed Development

Pod formation is only one step that must take place prior to harvest. The soybeans themselves don’t start developing until the pods have more or less grown to their full size. 

Seed Development

As I mentioned earlier, each soybean pod contains three individual seeds. These seeds start developing when the pod is in the ‘full pod’ stage of growth. 

From the ‘full pod’ stage, it takes about ten days on average for the seeds to reach ⅛ inch long. It then takes another 15 days on average for the seeds to grow to their final size, filling out the pod entirely.

8. Maturation

Once the pods and seeds are fully formed, they still need to mature on the plant. It takes another 30 to 40 days on average for 95% of the seed pods on a plant to mature. At this point, the soybean’s life cycle is considered complete from the perspective of a farmer or home grower.

When to Harvest Soybeans

I’m willing to bet most home gardeners will want to harvest their soybeans at the immature stage. These pods are ideal for edamame and similar dishes.

Harvest young soybeans when the pods are 2 to 3 inches long and bright green in colour. The seeds should be plump but have some give to them.

Mature soybeans (sometimes called dry soybeans) are harvested up to 3 months later. Harvest mature pods when they are brown and dry, and the rest of the plant has turned yellow. 

When to Harvest Soybeans

If you have enjoyed this article, here’s a link to Corn Growth Stages that you may also find interesting.

FAQ Soybean Growth Stages

How many months do soybeans take to grow?

Soybeans take 3 to 5 months on average to grow from seed. Keep in mind that soybeans grown for edamame (i.e., picked when the pods are still green) may take slightly less than three months to be ready for harvest.

Can you grow soybeans in the same place each year?

Though farmers tend to rotate their soybean crops to preserve soil quality over time, this isn’t always necessary in the home garden. If you have the space, however, it’s always best to rotate your edible crops to prevent the spread of disease and related issues.

Citations

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Ben's horticultural interest grew when graduating from Hertfordshire University in 1997. Having contributed to numerous publications including Better Homes & Gardens, Garden Design Magazine, and The English Garden. He is also the author of Propagating Houseplants Made Easy.