The Plant Hormone Auxin

The Plant Hormone Auxin

Did you know that plants have hormones too? Plant’s hormones control their growth, reactions and even their movement. Perhaps the most interesting and widely studied plant hormone is Auxin. Auxin was discovered in the 1930s by Charles Darwin. In 1880 his book “The Power of Movement in Plants” describes the effect light has on a plant’s movement. If you’ve ever noticed a plant growing towards the light then you’ve witnessed auxin in action!

Plants move…

Tropism is the technical term for the response of plants to environmental or physical stimuli and this movement is possible because of auxin. There are many different forms of tropism. Phototropism is when a plant responds to light, like your houseplant growing towards the bright window. Geotropism is a plant’s response to gravity. Plant’s roots are positively geotropic and their shoots or stems are negatively geotropic because they grow upwards, against gravity. Thigmotropism is a plant’s response to touch. Imagine ivy growing on a fence, its tendrils are positively thigmotropic because they grow towards something they can grasp. Some plants are more thigmotropic than others. If you’ve never heard of the Mimosa plant I strongly suggest you check it out on youtube. You might even find some in your front yard.


The hormone auxin is also extremely important for propagation. Auxin promotes adventitious root formation. For example, when you cut a stem off a plant it will form very small roots in an effort to stay alive and take root somewhere else.  In the nursery industry, high amounts of auxin can be used to stimulate adventitious root formation during tissue culture to mass produce plants. One small shelf of petri dishes can yield thousands of plants.

As a gardener, Auxin is my favorite hormone to utilize when propagating. Next time you’re at Home Depot, purchase some rooting hormone powder from the nursery. The main ingredient is auxin and it’s a very reliable way to ensure your cuttings form roots. Just cut a small stem or leaf off your favorite plant, dampen the end with water and dip it in the rooting powder. You can leave the cutting in a damp paper towel near a window indoors or immediately place it in soil. Now you can buy one plant and create 20 more on your own. No green thumb is required, give this tried and true method of propagation a shot!

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