Properly harvesting basil is essential in order to get the most out of the herb. Basil plants need to be harvested frequently both to keep them from flowering and to promote new growth. Basil plants can grow quite large, and it is possible to cut off a large section of the plant at once, but this can only be done once or twice per growing season. Most planters chose to harvest a modest amount once a week or once every three weeks. Utilizing proper harvesting technique can yield 10 or more cups (2.4 liters) of leaves per season.
A basil plant needs to grow large enough to establish itself before it can be harvested. Generally, once the plant is about a foot tall (0.3 meters), it is strong enough to handle harvesting, though counting the leaves is the best way to determine whether the plant is ready. The leaves of the basil plant grow in pairs, and the plant will flower once it develops six pairs of leaves on a single stalk. Harvesting basil each time there are four pairs of leaves will yield the greatest crop. Allowing the plant to go to flower will have a substantial effect on the taste of the leaves and should not be allowed.
Once there are four pairs of leaves on the first stalk, the gardener can pinch off the top of the stem just above the lowest set of leaves. After harvesting basil for the first time, there will be only two leaves left on the primary stalk. As long as the leaves are taken from directly above a leaf node, the the single stalk will split into two branches. These two branches will then grow separate sets of leaves which can themselves be harvested once the plant develops four sets of leaves on each of these stalks. It is possible to harvest at three or two sets or leaves on each stalk, though it is important to leave one set behind on the plant.
As the season progresses, the basil plant will bush out if it is harvested in this manner. Gardeners that find a large plant too unruly to manage can remove half of it at some point in the season. If the plant appears healthy, culling it in this way should not damage the rest of the basil plant. Harvesting basil should continue throughout the growing season.