Plant auxins are hormones that help guide plant growth. They promote cell expansion and can inhibit flower development. The most common and important of the auxins is indole-3-acetic acid (IAA). Auxin concentration on the far side of a semi-shaded stem causes cells to enlarge so they push the leaf to face a light source (phototropism). The ratio of cytokinins (opposing hormones) to auxins will either promote root growth (if the ratio is auxin rich) or promote shoot growth and inhibit root growth (if the ratio is cytokinin rich).
The primary (but not only) flow of the IAA auxin is from the growth tip down through the phloem (transport tubes in the stem) toward the root tips. For example, the growth tip on a cutting sends auxins down to encourage root formation at the lower end of the cutting. Once established, the roots will send cytokinins back up to the growing tip to promote leaf, stem, and branch development. Rooting products generally allow the addition of auxins where they are needed, namely on the stem where root development is desired. Along with natural auxins, these hormones encourage the undifferentiated meristem cells to become root cells, forming first callouses and then roots on the stem.
The terminal bud is not inhibited by auxins, but it produces auxins that cause axillary buds to remain dormant. This is why in some plants the terminal bud tends to produce the largest fruit or flower. The effect is cumulative, so each higher growing tip in a line adds to the effect of those below. As a result, the outermost growing tip on a branch tends to have a progressive advantage over budding sites closer to the stem. As a memory aid, one might picture a large ox (auxin) playing “king of the hill” where it dominates those below it. In an indoor garden, auxin flow is sometimes manipulated to provide a more even canopy.
Methods of pruning and their effects
Pinching — A common tactic in pruning is the removal of the top growth tip or the last growth tip on a branch which also removes the auxins they were producing. This gives the bud nodes directly below it the advantage of not being hindered, allowing them to develop larger. This is commonly known as pinching because the tips are pinched off, allowing the next growth tips in line to develop. Pinching is generally most effective in plants with pairs of budding nodes on either side of the stem. The cut should be made just over the next two nodes in line as any higher stem will die off after the procedure.
The energy that would have gone into increasing the size of the original single tip is divided between the new two topmost nodes. If the two nodes are then allowed to develop and they, in turn, are pinched, then the number of branches will again double to four. This can be repeated as desired until the plant is sufficiently bushy. In some plants, distributing the energy among more tips this way can result in several smaller similar sized fruits or flower clusters instead of a few large ones.
Definitions to know
FIM (F*CK I Missed) — This variation on pinching involves removing enough of the top growth tip to retard auxin development and growth without removing the entire tip. This allows for the next growth tips in line to catch up to it while it recovers. This technique is especially beneficial when growing under artificial lighting, which has limited penetration.
High-Stress Training (Supercropping) — The growing tip need not necessarily be removed to take advantage of auxin manipulation. If the top of the shoot is bent so it is no longer taller than the next nodes, then it loses much of its advantage over them. This is done by carefully weakening the stem by slightly crushing it between forefinger and thumb, and then bending until the branch is in a horizontal line with the nodes to be encouraged. This method does cause damage and stress to the plant, and if done incorrectly can result in breaking off the branch being treated.
Low-Stress Training (LST) — As the plant grows, new growth is trained as desired while it is still pliable. This avoids the stress and damage associated with high-stress training while providing a similar effect. Training a climbing vine to a trellis is an example of LST.
Determining which method is best depends on the situation and desired results. Not all plants react the same and some types of plants are more easily influenced than others. Both pinching and LST have been popular ways of manipulating plant growth for a long time, well before the discovery of auxins. But understanding the connection between auxins and plant growth is helpful when making decisions about pruning plants, training branches, or applying a rooting compound.