Most people agree that sweet corn is a seemingly simple crop, but in reality, it has dynamic growth and reproductive stages. The yield and quality of sweet corn are directly influenced by these growth and maturity stages. If and when these stages are interrupted or subjected to adverse conditions, it can affect the growth of the stalks and the development of the ears. This guide is meant to help sweet corn growers understand what is happening during each corn growing season, why these stages are so critical, and how to avoid stress factors that can lead to potentially low yields or poor quality.
How Sweet Corn Grows
From the time a corn seed sprouts until the stalks form ears, sweet corn plants go through about 18 different vegetative (V) stages, followed by 6 main reproductive (R) stages. All these growth stages must happen without interference or stress for the crop to produce quality ears of corn.
- All sweet corn growth stages must be completed in a specific number of days, with each stage corresponding to physical developments, which can vary depending on environmental and growing conditions.
- The yield & quality can be affected or altered by any number of stresses during their short and formative development stages. Meaning, one hot spell, a lack of soil moisture, or a nutritional imbalance at the wrong time, can easily result in no crop or a poor crop. This is why sweet corn crops are not as simple as they seem.
- All sweet corn varieties must go through the same growth stages to produce quality ears. Unfortunately, stressful conditions can occur in any region to any crop. Many different sweet corn varieties exist because many regions and localities have quite variable growing conditions and soils. Finding the sweet corn that performs best in your growing conditions and environment may take some time and trial & error.
Growing to Reproductive Stages
Stages start at VE (Vegetative emergence), grow through a total of about 18 development stages (V18), and culminate at the VT (Tassel) stage. Then the sweet corn plants transition into several reproductive stages - R1 (Silking), R2 (Bister), and eventually reach the R3 (Milk) stage which is the optimal stage for harvest. Often, sweet corn crops go through these stages without a hitch, but we never know what weather conditions during any season will bring.
In optimal conditions, it only takes about 9-12 weeks for sweet corn to grow from the VE stage to VT/R1 (Silking) stage. This means a lot of growth and change happens in the corn crop during these few weeks of vegetative growth.
- The V6 to V18 growth stages (approx. 30-50 days) determine all the critical ear development factors like the number of ears, ear size, and number of kernels per row, along with stalk elongation and tassel & pollen development.
Importantly, ear development, including quality characteristics is all determined many weeks prior to tassels or silks forming. For example, crop management practices that reduce plant stresses during V10 to V17 growth stages are most critical to help maximize the potential for yield and quality of sweet corn. If adverse conditions stress the crop during this critical timeframe, ear development can be affected and potentially even halted. Unfortunately, even if the crop becomes stressed, the growth of the stalks usually continues with little to no visual evidence of distress or trouble. It is only when the stalks fail to form ears that the grower realizes the plants had become stressed at some point and by this time little to nothing can be done to salvage a crop.
Plant stress can occur from many factors. The factors growers have control over include the soil nutrition or pH, watering, and application of pesticides if needed. Unfortunately, growers have virtually no control over most of the major factors that cause plant stress and lead to growth issues.
- Fluctuating temperatures
- Excess rain or moisture
- Insect or disease pressures
Individually or in combination, any of these factors can adversely affect corn growth and influence ear development.
Development to Harvest
VT stage = Tassels
Tassels typically appear a few days before silks emerge. The time between these two important stages will depend on the weather and field conditions. This pattern of weather and field conditions affecting growth continues until ears are or are not formed. It should not be a surprise that if conditions prevent proper tasselling or silking, ear development or the quality of the ears is at risk. By this time in the growth of the stalk, it is too late to avoid problems. This is why it is so crucial to prevent plant stress earlier in the growing season.
- Growers must be aware that sweet corn can tassel without ear formation. Tasseling does not indicate that all conditions were or are perfect. Few to no ears (barren plants) can happen due to several different factors including late emergence, high plant densities, early season temperature fluctuations, drought, excess moisture, or the ever-present lack of proper nutrition.
R1 stage = Silking
- When silks become visible outside the husk.
- Pollen shed from the tassels happens mostly in the morning and evening hours, with pollen grains being deposited onto the silks which then fertilize the ovules.
- Each ovule produces 1 single kernel.
- Moisture stress during this stage leads to desiccation of the silks or drying of the pollen. Both of which can cause reduced kernel set, kernel less tips, or total ear loss.
- Higher than average temperatures during peak pollen shed periods can also interfere with this process.
- Windy conditions, heavy rain, or hail events from summer thunderstorms also interfere with pollen to silk transfer which ultimately affects ear quality.
- The pollination process takes about 24 hours but do make note of when this R1 stage occurs, to help gauge the optimal harvest period, since ears typically reach the milk stage about 18 to 22 days from pollination. To reiterate, the R1 stage is an important benchmark to watch for and take note of.
R3 = Milk stage
- The milk stage is when sugars, starch & moisture are all in balance within the kernels.
- Ears harvested before this stage will have small kernels, lack color, and be watery. This is referred to as R2- Blister stage, which occurs just prior to R3.
- At the proper R3-milk stage, silks should be dried and brown, if they are still pliable, green to golden the R3 stage has not been reached yet.
- Purple to reddish pigments in silks can be a red flag of some stress-induced conditions from drought, high temperatures, poor pollination, or even root restrictions. Not much to do about this when you find it, but don't be surprised if potential ear quality issues occur in the crop.
- Ears harvested after the R3 stage, kernels will be doughy due to rapid starch accumulation, and declining moisture levels that cause a lack of flavor with a hard, undesirable texture. This R3 stage is fleeting with sugars rapidly converting to starch. The speed of this conversion depends on the type of hybrid corn planted.
NOTE: Other than proper harvest time, the biggest factor influencing the flavor & quality of sweet corn is proper isolation to prevent cross-contamination from other types of corn. For basic guidelines regarding proper isolation, please review the Sweet Corn Garden Guide.