How a Cotton Plant Grows
Contained within the seedcoat of a viable cottonseed is a new plant waiting for the correct set of environmental conditions to occur to start it germination process. Man has taken the cotton plant for granted, enjoying its benefits without fully understanding its growth. The goal of this factsheet is to help you better understand how a cotton plant grows, so better management practices can be used to produce maximum yields.
Seed Germination and Seedling Emergence
Seed germination and seedling emergence are the foundation of your total crop. Understanding these activities will help you to lay the groundwork for a vigorous crop.
If you closely examine the inside of the cottonseed, you will find all of the essential parts that form the mature plant. There are two well developed cotyledons, that will form the seed leaves which manufacture food for the young seedling. Located between the cotyledons is a structure referred to as the epicotyl which is the shoot that will form the main stem. The next identifiable part is the hypocotyl, this is the first plant part seen above ground in the emergence process. The tip of the small seedling is referred to as the radicle; it is the first structure to emerge from the seedcoat and will ultimately form the root structure of the cotton plant.
These tiny parts make up the embryo or kernel. It lies quietly within the protective confines of the seedcoat. Once the seed is placed in the ground the miracle of seed germination begins. Moisture from the surrounding soil seeps through the seedcoat at the broad end of the seed, this is an area of specialized cells referred to as chalaza. The absorbed water follows the tissue around the embryo to the radicle cap at the narrow end of the seed. As it moves, the water softens and penetrates the tissues, and triggers a wide range of chemical reactions. By now,moisture is penetrating all parts of the seedcoat, and the swollen embryo appears ready to burst. The radical forces it way through the tiny opening at the pointed end (micropyle) of the seedcoat and pushes downward into the soil.
At the same time, the hypocotyl has begun to stretch and forms an arch or crook as it makes it way to and through the soil surface. If the seed has been placed in a firm seedbed the protective seedcoat will remain underground as the expanding cells straighten the hook and pulls the cotyledons and the epicotyl (shoot) free eventually lifting them above the soil surface. Exposed to light, the newly unfurled seed leaves turn green and begin to manufacture food–a short-term function that they will perform until the true leaves take over. Soon the bud above the cotyledons enlarges and unfolds to form the stem. The true leaves and branches will develop there.
Under favorable conditions, seedlings emerge can occur in 4 to 5 days after planting. Emergence will take longer if seed are planted in cool soils. Temperatures below 60°F are detrimental to germination, emergence, and seedling growth. During the first 60 to 100 hours of germination, the radicle tip is easily damaged by chilling, lack of oxygen in the soil, or too much moisture. If the tip is killed, a shallow system of secondary roots develops that makes the plant more subject to moisture stress later in the season.
As a rule of thumb planting should be delayed until the soil temperature at the eight inch depth averages a minimum of 60°F for 10 days (temperature should be taken at 8 a.m.). Allowing soil temperatures to increase before planting will cut the time needed for germination and seedling emergence and helps to ensure healthy, uniform stands.
It also is important to use high-quality seed and plant in a firm, well prepared seedbed with adequate moisture. Seed should be planted at a depth of 1 to 2 1/2 inches, depending on soil type and availability of moisture. Planting too deep can significantly reduce plant population and seedling health. It is better to delay planting than to plant seed too deep.
Getting A Stand Is Important
Insects and weeds may be the visible enemies of a good cotton crop, but what you do at planting time is even more important. The potential of a cotton crop is determined in the first 30 to 40 days after you put seed into the ground. Everything that occurs after you get a stand can only maintain or decrease yields. A “good stand” refers to the number of healthy, vigorous seedlings that are evenly distributed in the field. This may be 2 to 4 plants per foot of row depending on soil type, row width, planting date, and moisture during the growing season.
Getting a stand requires proper seedbed preparation, favorable soil temperature, proper planting depth, adequate soil moisture, high quality seed, elimination of soil compaction, avoiding chemical injury and protecting the plants from high winds, blowing sand, insects and diseases.
Actually, seed quality should be listed first. Poor planting seed is the primary cause of stand failure. Emerging seedlings are poorly equipped to withstand the challenges of diseases, insects, wind, and weather, including moisture stress and heat stress.
For that reason, you should use only seed that have a cool-warm vigor index of 155 or higher. The cool-warm vigor index is obtained by combining the warm germination test percentage with the cool germination test percentage.
Cotton should be planted in well prepared seedbeds that are firm, warm, and moist. Planting should be based on soil temperature and the weather outlook for a month after planting. It is generally recommend that you plant only after soil temperatures at the eight inch depth averages a minimum of 60°F for 10 days (temperature should be taken at 8 a.m..). Weather outlook is important because rain and cool temperatures following planting can slow germination and reduce stands.
The same factors that delay germination and seedling growth encourage seedling disease and insect problems. The first challenge encountered by the developing cotton plant is seedling disease complex made up of one or more soil borne fungi. You may know these seedling diseases by more familiar names: Pythium, Rhizoctonia, and Thielaviopsis. Treating planting seed with fungicides often helps ward off seedling diseases.
Don’t plant seed too deep in soils that are overly wet, cold, compacted, or high in chemical concentrations. It’s important to apply herbicides as indicated on the label to avoid root pruning or seedling injury. Fertilizer needs to be applied in a manner that reduces the potential for seed and plant injury.
Early season insects such as thrips, cutworms, leafminers, and aphids are a concern. Seed treatments, in-furrow applications of fungicides and systemic insecticides (if needed), and foliar insecticides applied when threshold levels are reached can more than pay for themselves in helping get your crop off to a quick, health start.
|Here’s where a good stand starts–with high-quality seed. Micro-organisms in the soil and on and in the seed can cause seed rot or decay. Note visible mold growth on rotted seed in center.||Thrips distort and crinkle seed leaves (left leaf), slowing down early growth of the plant. This complicates postemergence weed control and may delay fruiting and crop maturity. Leafminer (right leaf) seldom does economic damage. Beneficial ladybug (center) feeds on eggs and small larvae of damaging insects.|
|Rain shortly after seedling emergence and cool temperatures often stimulate fungi that cause postemergence, damping-off. Seedlings are stunted, then they wilt and often die, leading to skimpy stands.||Early plant growth is slowed by excessive concentrations of preplant incorporated herbicides. The most obvious symptom is the thick pruned lateral roots.|
Building the Framework-Roots, Stem, and Leaves
The cotton plant is constantly manufacturing new, specialized cells to form the organs that carry out growth and reproduction. The four organs are the roots, stem, leaves, and fruits (squares, flowers, and bolls).
The roots are both the anchor and the life-support artery of the plant. They form the foundation that holds the plant firmly in place, and they channel moisture and dissolved nutrients from below the ground to the manufacturing system above. The cotton plant has a primary (or main) taproot with many branches (lateral roots) and along the lateral roots are root hairs. Root hairs are responsible for most of the moisture and nutrient uptake of the plant.
After seed germination the taproot grows downward for several days without branching. It may reach a depth of 9 inches by the time the cotyledons (seed leaves) have emerged from the soil. Branching of the taproot begins about the time that the cotyledons are lifted above the soil surface and the seed leaves begin to unfurl.
Soil type and texture, moisture, and aeration determine how deep taproots penetrate. A few will grow as deep as eight feet. Normally, however, about half of the total root length is confined to the top two feet of soil.
The mass of roots, large and small, that branch from the taproot make up the main absorption and anchoring structure of the cotton plant. Their distribution depends on a combination of weather, plant, and soil factors. Roots grow most rapidly when there is enough, but not too much, moisture; when there are no compacted soil layers; and when other environmental conditions are ideal for plant growth.
Tillage operations, proper use of fertilizers, and adequate soil moisture can improve conditions for healthy root development.
The basic root system normally is in place by the time the plant begins blooming, or by about 8 to 10 weeks after planting.
If the roots are the foundation of the plant, the stem is the framework that supports the vegetative and reproductive organs. It also serves as the distribution system–carrying moisture and minerals from the roots to the leaves, and food to the various parts of the plant.
A fully developed cotton plant has a prominent, erect main stem consisting of a series of nodes (branching points) and internodes (stem area between nodes). The cotyledons are the lowest two leaves on the stem and the only leaves directly across the stem from each other. As the plant grows, new nodes and internodes form. The first leaf to develop above the cotyledons is referred to as the first true leaf.
The time between node development is impacted by temperature and generally ranges between 3 to 5 days. A single leaf forms at each node in a spiral arrangement. In upland cotton grown in Texas, each new leaf commonly develops three-eighths of a turn above the preceding leaf. The course of the spiral may be clockwise or counterclockwise.
At the center of this growth activity is the terminal bud. It is at the fork of the main stem and the petiole that supports the blade of the top unfolded true leaf.
The terminal bud controls the upward pattern of stem, leaf, and branch development. If it is damaged–by hail, insects, or mechanical operations–the entire growth sequence of the plant is upset. The branch below the terminal bud will take over as the main stem, but it generally is weaker. The branching arrangement of the plant will be irregular, and growth will be delayed.
The leaves are the factory. Through the plant process of photosynthesis, leaves use the fuel of sunlight to convert water, carbon dioxide, and minerals to the sugars, starches, and proteins that the plant needs to survive, grow, and reproduce.
Leaves may vary in size, texture, hairiness, and green color, depending on the variety. However, weather conditions and cultural practices such as fertilization and irrigation management can also influence size, thickness, and color of the leaves.
|As the plant grows, internodes continue to extend, and new leaves are formed at each node. The number and length of internodes, which determine plant height, are influenced by the variety and also by soil type, moisture, nutrients, insects, and diseases.||The terminal bud is the key to plant growth and leaf and branch arrangement. If the bud is damaged, the normal development cycle of the plant is thrown off balance.|
Vegetative and Fruiting Branches
When the cotton plant is four to five weeks old it will expand its basic framework of root, stem, and leaves and begin to form vegetative and reproductive (fruiting) branches.
Vegetative branches primarily produce more stem and leaves. A certain number of leaves are necessary to carry out photosynthesis or production of plant food. But too much vegetative growth siphons off valuable energy and food that are needed to produce fruit. It delays your crop and makes pest control and harvest harder. There must be a balance between vegetative and reproductive growth. A branch develops from a bud formed in the angle between the leaf stem and the main stem node to which the leaf is attached.
Vegetative branches usually develop only at the lower nodes of the main stem (nodes 4 through 7). They grow nearly upright; and like the main stem, each has a single terminal bud at its tip.If the terminal bud of the main stem is damaged by insects or hail, one or more of the vegetative limbs near the tip takes over as the main stem. If the injury occurs early in the plant’s development, the entire plant may become bunchy and unproductive. That makes protection of the plant from excessive damage by early season insects very important. The cotton plant now makes a transition from vegetative to reproductive and begins to develop fruiting branches. Short-season varieties may set the first fruiting branch at the fourth or fifth node. Longer-season varieties usually form first fruiting limbs at the sixth to eighth node.
In addition to genetic differences, plant population, temperature, and stress also influence the location of the first fruiting branch. Cotton plants growing close together will have fewer vegetative branches and lower fruiting branches than will cotton plants spaced out farther in the row.
Excessive soil moisture and too much nitrogen early in the plant growth period also will cause the plant to set its first reproductive branch too high on the stem. The higher the first fruiting branch, the longer the plant will take to complete fruiting and to mature its bolls. On the other hand, moisture stress early in the season can result in the reproductive branches beginning at nodes 4 or 5, generally resulting in a smaller plant overall with reduced yield potential.
After the first reproductive branch has formed, new branches will develop every 3 days approximately. Unlike vegetative branches, fruiting limbs do not have terminal buds. Each new internode on the branch results from a bud that is formed in the axil (junction) of the leaf on the branch. Here’s what happens: The first part of the reproductive branch to become visible as the branch develops from the axillary bud is the floral bud; it forms the square.
As the branch grows and the internode lengthens, the square is moved away from its original position next to the main stem. A leaf develops beside the square but remains very small for four to seven days. Then, as this leaf enlarges and unfolds, a new axillary bud is formed and develops with its own floral bud to form the second internode and square of the reproductive branch. The process continues over and over throughout the season–leaf, axillary bud, internode, square, leaf, new axillary bud, internode, square, and so on. This type of growth results in the distinctive zigzagging form of reproductive branches.
Vegetative branches do produce some squares. But the process is much slower and very inefficient. So it’s important that your management practices stimulate and protect the early reproductive growth of your crop. The results should be a larger, and certainly earlier, crop.
|Heavy lygus damage promoted thick, bunchy growth in this cotton. This field should already be at the peak of flowering, but no flowers are visible.||Damage from insects or hail early In the season can disrupt the normal branching and fruiting pattern of the cotton plant. Early thrips damage to the terminal bud caused excessive vegetative branching in this plant.|
Under normal conditions, you can generally find the first square on the plant five to eight weeks after the cotton is planted. Since the development of cotton is temperature driven the number of days before you find the first square may be 35 to 47 days after planting. With continuing favorable temperatures the first white bloom should be visible in approximately three weeks after the square is set.
The first square is formed on the lowest reproductive branch of the plant. This branch may be located at the fifth to the ninth main stem node, depending on variety and environment.
If you don’t have squares by the ninth node (as an average of the field), your crop may be in serious trouble. Several reasons may account for the delay, most of the time it is due to lower-than-normal temperatures or insects.
The critical period for producing squares is from June through mid-July. The rate of squaring should increase each week through the fourth week. This rate usually levels off during the fifth and sixth weeks, then drops sharply as fruit retention matches the plants ability to provide food for growth.
You need to avoid any situation that may cause the squaring rate to drop off sharply at any time up through the fifth week. Research shows that as many as 85% of the total bolls that eventually are harvested come from squares set during the first four to five weeks of squaring.
Some shedding of squares is expected. In fact, under the best management, the cotton plant will slough off 40 to 50% of all squares that it produces. The important thing is not to lose too many of the early squares. Extensive shedding–especially if it occurs early in the season–can upset the vegetable/fruiting balance of the plant and reduce yields. Experts suggest that plants should be holding no less than 60% of the early pinhead squares (1/8-inch in diameter).
The first three positions on each reproductive branch are the key sites for fruiting. They account for most of the yield. According to research, over 70% of the total lint is produced from the first square on each reproductive branch. To put it another way, the squares nearest the main stalk on each fruiting branch will make up over seventy percent of your total yield. The second series of squares accounts for another one-third or more of the crop. Those squares farther out produce less than 30% of the final number of mature bolls.
Square shed may be the result of insect damage or poor growing conditions. Conditions that can cause a plant to drop its squares include very dense stands, rank plant growth, extended cloudy weather, too much nitrogen, low root oxygen because of water-logged soils, hot dry winds and temperatures below 60°F for several nights.
Dense stands or rank growth shade the lower fruiting branches. They either stop growing or shed a large portion of their squares. Avoid planting too thick or fertilizing or irrigating excessively.
Damage from plant bugs (lygus, fleahoppers, and tarnished plant bugs) also can cause square shed. Inspect fields every three to seven days beginning at the pinhead square stage. This lets you monitor the squaring rate and status of both damaging and beneficial insects.
The cotton plant has a tremendous capacity to make up for square shedding. It is very forgiving of mismanagement, pest attack, and poor growing conditions–but to a limit. Using those practices that will stimulate a high fruiting rate and square set provide a stern test of your management ability.
|Here are the various stages of square development: pinhead and matchhead square.||Here are the various stages of square development: 7 days, 14 days, and 21 days.|
|Exposed square (floral bud).||Cross section of square.|
The cotton plant develops in an orderly, predictable pattern. If you are familiar with the fruiting stages, their sequence, and the time required for each stage, you can tell if your crop is on schedule. For example, you should spot the first white bloom 60 to 80 days from planting. That will be from 20 to 27 days (23 days average) after the square or bud develops.
It will take about 8 days between the opening of a flower on one fruiting branch and the opening of the bloom in the same position on the next higher fruiting branch. That’s known as vertical flowering. About 6 days pass between the appearance of two consecutive blooms on the same branch (horizontal flowering).
The cotton bloom is a perfect flower (see sketch). It has both male parts (pollen-producing stamens, each with a double-lobed anther) and female parts (stigma, style, and ovary) in the same flower. The ovary has 4 to 5 carpels or locks. Each lock contains 8 to 12 ovules that may develop into seed. The outside parts of the flower include the calyx with its five leaf-like divisions or sepals; three bracts; and five petals that are fused at the base.
The petals of American upland cotton flowers are white or creamy colored. Those of Pima or extra-long-staple cotton are yellow. The flowers open during the morning, and pollination usually occurs within a few hours. Pollen grains from the anther drop to the sticky surface of the stigma. Fertilization–the union of a male reproductive cell from a single pollen grain and a female cell in the ovule–normally takes place within 24 to 30 hours after pollination.
The fertilized ovule develops into a seed. Some of the ovules may not develop fully or are aborted. If a majority of the seed abort, the boll will fall off the plant within 7 to 10 days after flowering.
Cotton flowers usually are self-pollinated. However, bees or other insects may increase the frequency of cross-pollination. Temperatures above 100°F and moisture–rain or high humidity–reduce pollination. A bloom will not pollinate after the first day.
The creamy or white petals of the flower turn pink after 24 hours and shed within a week as the fertilized ovules of the ovary grow into a boll.
In most of the Cotton Belt, the effective bloom period occurs from late June or early July to mid-August. Water stress during this period will cause the largest loss of yields.
As has been noted, many factors influence fruiting, blooming, and shedding. These include variety, temperature, length of the growing season, soil moisture, fertility, insects and diseases.
It is especially important to keep the plant developing and holding its fruit early in the season. Research shows that in the Southeast and the High Plains, about 85% of the total bolls are set during the first three weeks of blooming, 10% during the fourth week, and less than 5% from the fifth through the seventh weeks.
In the San Joaquin Valley of California, 64% of the bolls are set during the first five weeks of blooming, 28% during the sixth and seventh weeks, and less than 8% during the eighth through eleventh weeks.
|This plant shows all fruit stages: squares, white blooms, and bolls. You also can spot the vertical and horizontal fruiting arrangement.|
The Miracle of Fiber Development
|Cross section of a full-sized boll shows seed, lint, and individual locks. Upland cotton varieties have four to five locks; each mature lock contains seven to nine seed.|
The growth of a seed into a bearing plant consists of a series of microscopic miracles performed by nature. None of these events is more remarkable than the development of the cotton fiber.Once the tiny ovules that will become the seed have been fertilized, the young boll grows rapidly. The cotton fiber develops from the tiny cells located on the outer surface of the seed. Keeping one end firmly anchored to the seedcoat, the fiber stretches out, growing longer day by day. It will reach its maximum length in 15 to 25 days (18 days on an average) after fertilization of the ovule.
Fiber length is largely controlled by the genetic code passed along by the plant’s parents. But length also can be influenced by the environment. Stress during this period, especially for moisture and nutrients may cause fibers to be shorter than normal.
When the strands have stopped growing lengthwise, they begin to fill with cellulose. A cotton fiber is like a hollow tube. Each day, successive layers of cellulose are deposited on the inner surface of the fiber wall in a spiral fashion. The amount and pattern of cellulose deposited determines fiber strength and maturity. Fiber strength is closely related to genetic makeup. Micronaire is influenced by environment and management more than by genetic makeup. Unfavorable growing conditions during the “filling” period may result in weak, immature fibers with low micronaire (a measurement of maturity). Cool nights late in the season extend the boll development period and this can have several undesirable results. The most costly impact is the slow down in fiber filling combined with poor layering of the cellulose.
In approximately 24 days the boll will reach its full size. An additional 24 to 40 days of favorable environmental conditions are needed for the fibers to fill with cellulose and the boll to open.
Not every boll that is formed on the plant makes it to maturity. If not enough of the ovules are fertilized, the boll will fall from the plant 7 to 10 days after flowering. Insects, will feed on and/or bore into the boll. Damage done to small developing bolls 1 to 10 days of age will result in the boll being aborted. After 12 days the boll generally remains on the plant and the damaged locks result in reduced yields.
Young bolls 1 to 10 days of age will be aborted when the demand by various plant parts for food, principally for carbohydrates, exceeds the supply. Older bolls shorted on needed carbohydrates will be smaller in size requiring more bolls to produce a pound of lint.
At some point late in the fruiting period, the cotton plant starts to “cut out.” Cotton has reached “cut out” when the top bloom–in the first position away from the stem–is within 5 nodes of the top of the cotton plant. The plants response to “cutout” is to use the produced carbohydrates to mature the bolls on the plant. No new fruiting branches or squares are formed. Yields will be reduced if cutout occurs too early.
First bolls generally begin to open 105 to 130 days after cotton planting. Bolls set later in the season often take 12 to 25 days longer to mature than do those set early and in the middle of the fruiting season.
If harvest-aid chemicals are applied too early, they prevent deposit of enough cellulose to produce a strong, well developed, mature fiber. Yields also may be reduced. An accepted rule of thumb is to defoliate when 60% of the total crop is open. Desiccants are applied when at least 80% of the boils are open.
|Bolls set in a timely fashion have time to develop and open to produce quality lint with good yield potential.||Late set bolls are generally smaller and may not open. Quality of lint and yield is generally low.|
Your ultimate goal: An abundance of clean, well developed, mature bolls.
This factsheet was inspired by the series of articles written by Del Deterling for Progressive Farmer in 1982. Del Deterling was assisted in that endeavor by Dr. EI-Zik (research scientist with the Department of Soil and Crop Sciences at Texas A&M University) and photographs were provided by Bobbe Baker (professional freelance photographer).
Photographs were obtained from a number of sources and sincere appreciation is expressed to all contributing photographers.
The information given herein is for educational purposes only. Reference to commercial products or trade names is made with the understanding that no discrimination is intended and no endorsement by Texas AgriLife Extension Service is implied.
Educational programs conducted by Texas AgriLife Extension Service are open to all people without regard to race, color, sex, disability, religion, age or national origin.
Issued in furtherance of AgriLife Extension Work in Agriculture and Home Economics, Acts of Congress of May 8, 1914, as amended, and June 30, 1914, in cooperation with the United States Department of Agriculture. Edward Smith, Director, Texas AgriLife Extension Service, The Texas A&M University System.