What Happens When We Prune?
By: Don Julien
Several events happen within a bush when we prune. The first is wound healing. The next is a change in the flow of auxins affecting apical dominance. The third is a change in the relative size between root system and bush.
When we prune a rosebush, we are “wounding” it. When we cut off a cane, we expose plant cells that were not designed to be exposed. Woody plants have a two step process for repairing damage from such wounds, protecting the plant from losing fluids and from invasion by pests. The first step is to harden the exposed cells. The second step is to regrow tissue from the cambium layer surrounding the wound. This new tissue is called a callus. This process is most visible on tree scars; the wound is a hard smooth area surrounded by a corky collar that over time grows to close over the wound.
In roses, we observe the hardening in a pruning wound, but we usually only observe callusing on the bottom of cuttings, the chunky white mass that forms over a period of two to three weeks. This first step of hardening begins within 24 hours. A number of changes occur inside the cells within a boundary zone at the wound. Starch granules within the exposed cells degenerate. Cells adjacent to the wound increase production of a number of endoplasmic reticulum, ribosomes, polysomes, cytoplasm and dictyosomes. What these are is not important, other than that they play a role in developing polysaccharides. The polysaccharides are deposited on the boundary cell walls, and precede the formation of lignin, a tough cellulose substance that hardens the cell walls. Meanwhile, abscissic acid stimulates an as yet unknown process that produces enzymes used in producing suberin, which also attaches to the cell walls, and has some action in controlling invading fungi.
Ligno-suberized cells begin to form within 3 to 7 days, although the healing process can take several weeks. Temperature and humidity influence the rate of healing. Wound healing proceeds rapidly at 50°F and a relative humidity of 80 to 100%. Lower humidity requires higher temperatures. Relative humidity below 50% will inhibit healing.
Changing Apical Dominance
Auxins flowing from the apical bud(s) higher on the bush inhibit bud break from dormant buds farther down the cane. Pruning cuts off the apical buds, interrupting the flow of auxins. The bud directly below the pruning cut begins to produce auxins, taking over the position as apical bud, controlling the buds below and retrieving cytokinins from the roots.
Pruning is our artificial means of controlling bud break on our bushes. Early growth that promises lackluster performance can be pruned away to dormant eyes that act as if the bush just woke up. Pruning can also cut away early growth damaged by late frosts. In pruning, we choose which buds will be apical; by selecting buds on one side or another, or higher or lower on the cane, we can shape the bush to our wishes.
Traditionally, rosarians in the Seattle area prune in mid-February, although those in the cooler foothills wait until early March, cutting away any early growth. Pruning to dormant buds in mid-February times bud break to early March when day-length, temperature and light intensity will all be suitable for encouraging sturdy new growth, resulting in blooms in early June. Some exhibitors use pruning to time the blooming to coincide with rose shows. They will prune their bushes at two week intervals through February and mid-March to time the blooms through a six-week period, early June through mid July.
Shrubs and Old Garden Roses merit a special mention. Generally, we recommend that once-blooming OGR’s be pruned lightly after blooming in summer. Both shrubs and OGR’s are pruned in such a way as to encourage twiggy growth on lax canes. Apical dominance is not as strong in this lax form as it is in the upright form of hybrid teas, floribundas and climbers. In spring, early bud break may produce some unproductive growth, but it is rapidly followed with a profuse succession of productive bud break, resulting in heavy bloom hiding any blind growth. Since these shrubs are already at their preferred size, shoot growth is short and quick to bloom. Although most OGR’s are once-blooming, the succession of bud breaks may result in bloom periods of up to six weeks. This is also dependent on weather; a warm early spring may encourage faster bud succession, earlier bloom and shorter bloom period.
Each rosebush has a preferred size, based on variety and root system. A general guideline is that the plant above ground will match the root system below. Assuming a plant is established, light pruning will result in lots of smallish flowers on short stems, because the bush is already maxed out to the size of the root system. A hard pruning produces big canes and few, but larger flowers because the bush is trying to recover to its preferred size. Some rosarians exert some control over the size of a rosebush, not by just pruning the canes, but by root pruning as well. This is done by cutting a circle around the bush with a shovel, cutting off lateral roots. Then cane pruning brings the bush size down to match the new root size.
The balance of auxins and cytokinins influences the development of either roots or shoots. High auxin and low cytokinin levels encourage root formation, while low auxin and high cytokinin levels promote shoots. Hard cane pruning reduces auxin production, stimulating new shoots. Root pruning reduces the number of sites producing cytokinins, stimulating root growth (and controlling shoot growth). The combination of the two prunings retains a balance, encouraging both roots and shoots, but at a reduced level.
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