Plants will stretch when they are receiving too much shade and when they are too hot. Be sure the greenhouse is not located where it gets shaded by trees or buildings, especially during the cloudy winter months. Full sun is required.
It's like mowing the lawn: cut the tops (and the roots, when you root prune a houseplant) when they get a bit larger than desired to slightly less than desired, let the plant grow again, cut it back again, and so on.
If your seedlings are growing tall and thin looking, the technical name for that is etiolation. The reason why it happens is that the plants do not have sufficient light in the right wavelength, so they stretch themselves upward in the hope of getting nearer to the sky and a source of strong light.
Pruning. If the crown of your tree is stretching too tall or wide, corrective pruning can help rein it in and size it down for the space available. It's crucial to prune a tree before it becomes a problem because taking away too many branches could cause unnecessary stress.
How to Prune. Any time you prune, make proper cuts so you won't damage your plant. All cuts should be made on the branch side of the stem collar, which grows out from the stem at the base of the branch. This protects the stem and other branches that might be growing, and allows the tree to heal more effectively.
It's like mowing the lawn: cut the tops (and the roots, when you root prune a houseplant) when they get a bit larger than desired to slightly less than desired, let the plant grow again, cut it back again, and so on.
I also hear from lots of gardeners who are worried about killing a plant by pruning, but the truth is that pruning encourages more growth and improves plant health. A cut to a branch causes a chemical response, triggering a plant to develop new foliage and branching.
When and How to Prune Your Plants: Top off your plants while they are young to encourage them to grow out, rather than up. The picture on the left is a pepper plant that I “topped off.” That means that I cut off the top of the plant to slow it's progress growing up, and to make the plant more bushy and compact.
Many "leggy" indoor plants can easily be made bushier by cutting back the long stems and encouraging shoots at the base of the plant. The best time is early spring just before new growth begins.
Although being tall is advantageous in light competition, it entails costs for plants. As leaf height increases, plants need to invest biomass more than proportionately in the stem to support their own weight, which in turn reduces the fraction of leaf mass in the plant (Givnish, 1982; Niklas, 1992).
Plants will stretch when they are receiving too much shade and when they are too hot. Be sure the greenhouse is not located where it gets shaded by trees or buildings, especially during the cloudy winter months. Full sun is required.
Pruning. Pruning is one of the most direct methods to control your plant's size and shape. Regularly trim back excess growth to maintain the desired size.
Extreme pruning can end up killing a plant. Better Approach: When pruning an overgrown tree or shrub, never remove more than one-third of the plant's mass during a growing season. Prune very overgrown plants over a span of three years.
It's healthy, but getting too tall. Once it's taller than the window the growing part of the plant won't have enough light. It's time to cut off the top and root it, making a new, shorter plant. To do this, you a sharp knife, rooting powder, a clean pot and fresh potting mix.
As plants grow taller, they sometimes need a bit of support (physical, not emotional). This is especially true of plants like monstera, which would grow as vines in the wild and use trees to support them. Solution: Put a moss pole in the soil and lightly tie your plant to it, to help it hold itself up.
You can trim a leggy plant back, and the plant can actually grow stronger after the trimming. Yearly cutting can also encourage the plant to send out brand new shoots, creating a beautifully thick and lush plant. Trimming, pinching, and pruning are all helpful activities for eliminating leggy growth.
Renovation pruning (also called renewal pruning) removes all stems down to the ground. A second way to prune overgrown, deciduous shrubs is to cut them back to within 4 to 6 inches of the ground in March or early April. This severe pruning will induce a large number of shoots to develop during the growing season.
For stem plants, you want to cut between nodes (the points where the leaves grow from). You can replant the healthy tops if you want. Stem plants tend to branch out from the point where they're cut, sending out two little shoots on each side.
During excessively cold temperatures
Plants are more brittle in cold temperatures. Messing around with them may cause branches to snap and split unintentionally. It's minor, but it could impact the look of a plant or create a haven for insects and diseases later. Don't prune when the temperature falls below 25°F.
At this point in time science shows no evidence that plants suffer from pain. There is some evidence they react to being cut, but this is a basic stimulus response that is similar to how they react to sun light. It's not painful for them, it's just like a mobile phone that flashed low battery.