Cotton is grown commercially in many parts of the world with long, warm growing seasons. If you live somewhere too cool to grow cotton in fields you can still plant seeds inside and transfer the seedlings to larger pots outside during the warmest months. It takes at least 140 days for a cotton seed to grow into a plant with a mature boll. The bolls are full of seeds for next year's cotton crop.
Not all cotton plants produce white bolls. Experiment with the natural green and brown varieties too. Erlene's Green is an organic cotton seed with light olive-green fibers; Arkansas Green has soft light-green fibers; and Mississippi Brown has light-brown fibers. Red Foliated is a white cotton plant with red stems, flowers and leaves. It grows to less than two feet tall, so is a good choice to grow in pots.
In the spring, plant your cotton seeds in 6-inch pots with moist potting soil. Fill the pots with the soil, and poke three or four seeds into the soil about an inch deep in each pot, being sure to give each seed its own space. Cover the seeds with soil, and keep the soil moist but not soggy. You'll see seedlings in a week or so. Rotate the pots to keep the little plants from leaning over too far for light.
If you don't live somewhere with a long, warm and sunny growing season you will need to create the ideal conditions for your cotton plants. For the best results, transplant each seedling into a big pot (at least 30 inches deep) nearly filled with good, fertile soil. Place the potted plants in a warm, south-facing spot outside as soon as the weather permits.
Once established, cotton is a drought tolerant plant, so let the soil dry out before watering. In five or six weeks you'll see the first buds, and in another three weeks or so the buds will bloom into flowers.
The cotton flowers will turn red and drop off the plant, leaving behind a cotton boll, or pod. Leave the boll alone until it bursts open. Stop watering. The plant dries out and withers in the sun. Your cotton is ready to pick when all of the cotton bolls have opened and the cotton fiber inside has dried so it has a fluffy texture.