Thursday, February 10, 2011

The Science Behind What Grandma and Mom Were Doing.

Did you ever wonder what was happening as Mom was hacking and whacking at her petunias? Curious as to what was being achieved by Grandma lopping off new shoots from the apple tree? There are many things in life that we do to achieve a certain result without knowing the actual mechanisms in play.

Pruning and pinching are regular tasks in growing many flowers, ornamentals, vine crops, and trees. You could even look at mowing your grass as a form of pruning. More interestingly let’s look at what this does to the hormonal balance of the plant. There are five key hormones in plants, in two separate groups:
·                     Growth hormones – Cytokinins, Auxins, and Gibberellins.
·                     Stress hormones – Abscisic acid and Ethylene.

When we are pruning, pinching, and mowing we are influencing cytokinin and auxin ratios in the plant. Cytokinins are produced in the actively growing root tips and move upward in the plant stimulating processes such as shoot formation and branching. Auxins are primarily produced in the actively growing shoots and leaves. It is the ratio of these two hormones to each other and not the amount of either that matters. Both are required for cell division to take place. A higher amount of cytokinin coming from vigorous root growth will cause the plant to respond with vegetative growth to produce more auxin. When you prune or pinch a growing point and leaves off of a plant you are removing an auxin factory from production.

In the case of a rapidly growing vigorous plant with lots of auxin factories, pruning can be a good thing. Too much auxin production can overpower and inhibit cell division in the roots leading to a decline in root vigor, and hastening senescence and death. As auxin levels increase in the top of the plant and move downward it also causes dormancy in vegetative and reproductive buds. We would call this a top dominant situation, and when you cut off some of the auxin factories you are helping to return the plant to a more balanced root dominant situation.

In the case of a slower growing less vigorous plant, pruning can be a bad thing. Too little auxin production where very little auxin is available to make it down to the roots also limits root growth because cell division for new root growth will not be supported. Remember that root tips are only alive for a week or two, then they become a part of the “plumbing” and new root tips must be produced. 

When vegetative top growth outpaces your root growth (top dominance) you can have a beautiful plant with little endurance for production or stress. When there is a healthy, actively growing root mass under a plant during the entire season (root dominance), the plant can survive stress events and have good production. Four out of the five hormones controlling plant use of nutrition and governing how the plant grows are primarily made in the actively growing root tips.  Plant hormones must be available in sufficient quantities throughout the entire life cycle of the plant to maximize genetic expression and minimize stress.

By physically pruning/pinching shoots and branches Mom and Grandma were controlling the hormonal balance of the plant.  Most often they were putting the plant back into a balanced situation. Interestingly the same results can be achieved through nutrient support of hormone production, and sometimes by applying the hormones themselves.

Written by Brian Weems