Apical Dominance

This is the time of year to go outside and check on how your plants are doing. One thing that is super easy to do and will give lots of return for hardly any input is nipping off the tip of terminal shoots. It is so easy to do and will bring bushier plants and more flowers quite quickly.

Plants have a growing tip that contains a bunch of cells called the meristem. One location of meristematic cells is in the growing tip of the shoot. As the meristem differentiates and turns into more cells behind it, the tip grows forward. Along with growth, the area of the tip is the source of production of a whole class of plant hormones called the auxin group. There are many different types of auxins and they have many effects on the plant, but today we are learning about apical dominance.

The apex of the plant is the tip of the growing shoot. At the time of seed germination, the plant has to grow a stem and some leaves and start photosynthesizing. It must send up a shoot, first thing, to get everything going. As the shoot grows, a matching set of supportive roots must grow. Auxin is the hormone which tells the shoots to carry=on and extend, and the roots to get growing and proliferating. Auxin says, “I am sending out this shoot and I need lots of roots to feed it.”

Auxin also suppresses the growth of buds along the stem behind it. Plants make different amounts of auxin depending on their growth habit. A plant that grows mostly as a vine produces lots of auxin which suppresses the growth of branches and buds below it and allows it to elongate into one long growing shoot which we call a vine. You notice that vines are vine-shaped because they do not have a lot of branches, just lots of long terminal growth.

Vines are the extreme of the plant world when it comes to apical dominance. They stretch out and grow few growing points and each tip produces lots of auxin. If you nip off the growing shoot, the source of the auxin production is gone and the suppression of the lower-down side shoots and buds is gone. When suppression is gone, many growing points start to form and grow and the plant takes on a bushier nature.

All plants produce auxin at the terminal point so pinching that off will stop the growth of the branch you nip and allow side buds to grow. Since most plants produce flowers and fruit from the side branches, you can make a plant grow more compactly and flower more by pruning the tips.

But what about the root growth, you say, doesn’t the lack of auxin reduce the growth of roots? Yes, it will a little bit. You should wait for the plant to become established and medium sized before doing much pinching of apices. After it is growing well, it can handle the pinching and will do fine.

After a pinch two shoots will grow and the plant will be twice as bushy.


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