Size of Trees and Root to Shoot Ratios

Q. I have a large old 40-foot oleander tree. Every year as it begins to flower, some of the leaves turn yellow and drop. I realize I may need to fertilize but I was also thinking of cutting the trunks at about 20 feet, just below where it branches to encourage new healthier growth. Do you think this is an option? Obviously, I would do it late summer when flowering has finished.

Not a 40 foot oleander as the readers says but this oleander tree is about 18 feet tall.

A. It’s a temporary fix. There is such a thing as “root to shoot ratio”. What that means is that the top of the tree returns to its pre-pruning height as quickly as possible. It’s because of the size of the roots. The energy for growth is funneled into the top of the tree because the roots are at a maximum size for its height. Once the tree “catches up” to the size of its roots, it will slow in its growth because its nutrients for growth are partitioned once again.

Persimmon pruning and root to shoot ratio. This persimmon did not flower or fruit for two seasons until top growth (shoots) caught up with the root size (roots) demonstrating the importance of root to shoot ratios.

            Your oleander needs more water as the summer approaches; four times more from winter to summer and even more as it gets bigger. You can add more water without increasing the minutes by adding or changing drip emitters. But the nice thing about most oleanders is that they handle desert heat well, unlike some other “trees” that get bigger. Oleanders are Mediterranean trees in their water use, not “desert” trees.

December and January plant water use is about 400% more in the summer of June, July, and August.

We have found that giving a tree, growing in a moat six feet in diameter, about two inches of water gives it enough water until its next irrigation. The difference in water use of plants from winter to summer is about 4 times. The number of times that is done in a week or month depends on the time of year and whether it is a “desert tree” or not. At that size, if enough water is applied (so that its roots are wet to a depth of about 24 inches) will last until the next irrigation.

This is about a 15 year old nondesert tree watered with a bubbler and basin (about 15 minutes using pressurized irrigation) at about half of its canopy size. That same tree would require about 7 or 8 drip emitters and watering perhaps two hours to achieve the same thing and using the same amount of water.

What I can’t tell you is how many minutes that takes with drip irrigation. That needs a “gallons to minutes” conversion. In Las Vegas, mesic (nondesert) trees are watered about three times a week in the summer. In the winter they are watered a week or ten days apart, or about four times as long as during the summer months. When changing the irrigation time, focus more on the number of days per week or month rather than the minutes of applied water. It is too confusing. The number of minutes to water is seldom changed with a seasonal change in watering.