A common concern we hear from customers is that their plants are stretching: getting tall and skinny, bending or falling over, random branch spurts that lead to an uneven canopy, large spaces between internodes (which will cause a lower production of fruit due to less branches and flowering sites on a taller plant). There are many causes of this, the most common being:
- 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, uneven growth and far internodal spacing.
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.
- Heat. Plants tend to stretch (or "bolt") when the temperature gets too high for them. They do this in an attempt to cool themselves off: the more surface area they have, the more they can transpire. They can also grow odd-looking shoots at the tops in an attempt to protect themselves from the source of the heat (outdoors: the sun, indoors: your grow lights), and then eventually go to seed. Never let your garden get too hot or let your lights get too close.
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!
If you take all the precautions against the three things listed above and your plants still stretch, an easy thing you can do that will help to direct the plants to grow wider, rather than taller, is something called "topping," or "cropping." Once a plant is developed (roughly 3-4 weeks old and still in the vegetative stage of life), cut the main central branch at the top of the plant. This will redistribute the growth hormones to the entire plant and also send a signal that there is something at the top of the plant that will harm it. They will then grow outward more than they will grow upward.
There are also supplements that you can feed your plants that can help to limit upward growth, but be very careful when selecting these products if you're growing food crops. Most products that claim to keep plants compact, with more uniform fruit or flower development contain dangerous plant growth regulators (or PGR's) that were designed for the ornamental flower industry and are not safe for human consumption. Avoid the ingredients Paclobutrazol, Daminozide, Benzyladenine, among others. A safe alternative to those products would be a silicon/silicic acid additive like Cutting Edge Solution's Bulletproof Si, or a more concentrated product like Aptus's Fasilitor. While silicic acid is known for increasing cell wall strength (which helps plants to naturally fight disease, insects and stress), when used in higher than normal doses in the late grow/early flowering stages it can actually help to control upward plant growth.
All of that said, some plants just grow taller depending on their genetic makeup (for example, indeterminate tomato plants that continue to grow and yield vs. determinate tomato plants that reach a certain height, stop growing and yield in one cluster). There are tips, tricks and supplements that can help, but sometimes they just are what they are. If you have low ceilings/height limitations in your garden, try and grow plants that naturally stay more compact.
