Grow spearmint as you would grow any other member of the Mint family. It will tolerate many different growing conditions and grows well in nearly all climates. The plant prefers partial shade, but it can flourish in full sun to mostly shade. It also prefers loamy, damp soils with plenty of organic material. When growing spearmint for culinary purposes, fertilize with a well balanced fertilizer, organic composts or manure. Don’t forget to keep it well watered as it likes damp soils.
As any mint, spearmint can become invasive, so plant it in an area where it can be controlled and won’t crowd out other garden plants. Because it tends to take over the space, it is often grown in pots or planters or planted in a small pot and then that pot placed in the ground or inside a larger container. You can also use metal edging material placed around the plants to control them. Lift and replant your mint every 3 to 4 years, because if allowed to occupy the same plot of land year after year the leaves will become small and the stems wiry. If grown in pots you must repot your plant each year because the roots continue to grow and they run out of space very quickly. Take the plant out of its pot, divide the soil in few pieces and put each piece in a separate pot.
Like any mints, spearmint sends out runners that spread above and just under the ground, quickly forming large, lush green patches, so if planted in the right place it will make a pretty seasonal ground cover. Keep it under control by weeding out the stems that appear where they are unwanted. You can also plant it in tight places such as between pavers of a walkway where your feet will brush against the leaves to release its fragrance.
To harvest for culinary purposes, jut cut the branches, leaving a minimum of 1/3 of the branch, which will encourage the plant to bush out. Spearmint leaves can be used fresh, dried or frozen. They can also be preserved in salt, sugar, sugar syrup, alcohol or oil. The leaves lose their aromatic appeal after the plant flowers, so when the plants are breaking into bloom, cut the stalks a few inches above the root, on a dry day, after the dew has disappeared and before the hot sun has taken any oil from the leaves, and then dry them for culinary use for the winter.
Te have a continuous supply of fresh leaves you can force spearmint from November to May. This can be easily done by packing roots in a box or planting a few roots in a pot and keeping them moist at a temperature of about 15 Celsius degrees (60 F).
Start your spearmint patch with some transplants set out in the spring or in the fall in frost-free climates. Spearmint, like other mints, tolerates light frost, but the top will eventually die back in winter in all but the mild coastal climates. Roots are hardy in zones 5 though 9.
To propagate spearmint just cut off sprigs and put them in a glass of water. When cutting a sprig, cut about a centimeter above a junction. This way new branches will grow just below that place. After about a week, small white roots may appear under water. When the roots are long enough, take the sprig out of the water and plant it in another pot. Or you can just take cuttings and insert them directly into a potting mix. Water every day until the roots form and then regularly to keep the soil moist.
You can also propagate spearmint by sowing seeds indoors 8 – 10 weeks prior to last spring frost or directly outdoors in partially shaded, moist, soil when all frost dangers have passed. Sow spearmint seeds only just covered in loose soil. Transplant the seedlings when they reach 5 cm tall.