If this is performed low enough, the plant will actually develop four heads. Just be sure to leave four growing shoots. This is an area that is really misunderstood by many growers. While a single growing shoot will produce a larger single cola, topping a plant properly can put four colas in the sweet spot of light intensity. This allows more of the plant—especially the lower buds—to receive better light and produce larger buds. The overall yield from the plant is greater than an un-topped plant in most cases.
Not all strains benefit from topping, but most do. Some really short, slow Indicas such as Urkle just take too long to grow out when topped like this, but in general, this method allows us to consistently harvest five ounces per plant in soil. What I am saying is that all plants do better if topped but some slow-growing varieties will require additional time to mature to a proper budding plant size.
Seedlings are a bit different. You must allow the seedling to develop and have at least 4-5 internodes before topping. If you top a seedling too early, the shock will slow the plant down tremendously, wasting time and resources. Allow a plant started from seed to get herself rooted well and establish several sets of fan leaves. I like to wait until I see roots in the drain holes before my first topping.
Topping early is the key with most strains so that the plant maintains a low profile with the maximum amount of bud sites in the sweet spot. Topping also allows fewer plants to fill a room. When dealing with medical limits, growing fewer plants to produce the same amount of cannabis is a very crucial aspect to master.