- Not enough light, or the light is too far away. When plants aren't getting the light that they need, they stretch out in search of it. If your light source is too far away, your plants will grow quickly towards it, causing long, thin stems and branches that won't be able to support heavy fruit. With HID lights, a good rule of thumb is to keep them around 18" from the canopy. Fluorescent lights can be kept closer because they're not as intense and don't produce as much heat, so 7-12" is usually safe. A very basic way to judge where to keep your grow light is to put your hand at the height of the plant canopy. If your hand gets uncomfortably hot under the light, so will the plants. Raise it higher. If you notice any bleaching of the leaves, you know your light is still too close.
Tomato plants stretching for light: notice the thin stems and uneven growth.
The plants on the outside edges of this tent are much taller than those directly under the light because the light isn't spreading far enough to cover them, so they stretch to get closer.
This is supposed to be a tight head of lettuce, but apparently it got too hot.
- Over-crowding. Plants that are too close to each other fight for light and space, which leads to stress and an uneven canopy. A garden with less plants, where each one has room to grow and branch without touching other plants, will usually yield more than a packed garden. The actual number of plants isn't something to be too concerned about, quantity doesn't equal quality.
Here is a good example of an overcrowded grow tent of tomatoes: the canopy is uneven because they're fighting for space and light!