GARDENING: Even pines experience hormone problems |

GARDENING: Even pines experience hormone problems

Curt Swift
Free Press Gardening Columnist
When the needles on a pine curl, it is most often due to a tiny mite called an eriophyid mite, which disrupts the hormones of the tree.
Curt Swift |

Learning is a lifelong process and I’m one who enjoys learning and putting my talents and education to the test. Having years of experience combined with education certainly helps especially when I run into something I’ve not seen before. The other day I was introduced to a mature pinon pine with juvenile needles.

Seedling pinon pine trees have sharp blue awl-shaped needles distinctly different from the needles on a mature tree. The juvenile needles of a seedling pine cause people to mistake the baby tree for a blue spruce or seedling juniper. Further confusion occurs when the seedling grows clusters of inch-long needles. Did this young tree change from one genus to another; Picea to Pinus, or Juniperus to Pinus? Evolution doesn’t work that way, at least not in our lifetime.

Recently, I saw blue awl-shaped needles on the end of the twigs of a mature pinon pine. I knew something had disrupted the hormone status of the tree. Why else would the tree convert to its juvenile stage unless hormones were involved? Sometimes, when you shear a juniper, the resulting growth develops juvenile needles. This pine, however, had not been sheared so the question I had to ponder was what caused the hormone shift in the tree. On further examination of the area where this juvenile growth had developed I noted the twigs were slightly swollen.

This is where years of experience come into play. Pines infected with dwarf mistletoe develop swollen twigs and branches. The swelling is due to the development of roots of the parasitic mistletoe inside the infected twig or branch. The twig must swell to accommodate the roots of the mistletoe. There is no reason, however, for the mistletoe to disrupt the hormone status of the plant. Mistletoe has been parasitizing pines for as long as pines have existed and I’ve seen many pines (and junipers) parasitized by mistletoe but never seen juvenile growth result from the infection.

White Pine Blister Rust (WPBR), Cronartium ribicola, causes twigs, branches and trunks of pine to swell. This fungal organism is characterized by orange rust on the swollen tissue, but no rust spores were in evidence on this tree. WPBR has been identified in the Rocky Mountains infecting bristlecone pine (Pinus aristata) and limber pine (P. flexilis), but to my knowledge has not yet been reported on pinon pine. I concluded WPBR was not the cause of the swollen twigs on this tree.

I’ve seen twigs and buds of pine and spruce swell up when subjected to the misapplication of certain herbicides, especially auxin herbicides like 2,4-D. Auxins are plant growth regulators, i.e plant hormones. Further discussion revealed weeds near the tree had been sprayed with an auxin herbicide. The amount of herbicide absorbed by the tissue was sufficient to stimulate juvenile growth.

An imbalance in plant growth hormones can cause strange growths on trees. If you ski you have most likely seen masses of shoots in the top of the spruce trees as you ride up the lift. These witch’s brooms are due to a hormone shift or accumulation of plant hormones at the base of the cluster of crowded shoots. As you walk through a forest or town you will see large masses of tissue (swellings) on the trunks of trees. Whether caused by a bacterial infection, or mite or insect infestation, these burls are due to an imbalance of hormone(s). In these instances the change in growth pattern is easy to see. In some cases the change in growth is not as obvious, but just as noticeable when you know what to look for.

If you look closely at the needles of a pine tree you might see some of them are curling, just like a pig’s tail. If the petioles of a deciduous tree (a tree that loses its leaves in the fall) exhibit this characteristic, I know the problem is most likely due to an auxin-like herbicide that was applied too close to the tree. Either vapors or spray particles from a herbicide application drifted onto the tree, or the tree’s roots picked up the herbicide.

When the needles on a pine curl, it is most often due to a tiny mite called an eriophyid mite. This particular mite, in genus Trisetacus, transmits a hormone-like substance into the tissue as it feeds. This results in the needle twisting giving the appearance it was sprayed with a herbicide. If a herbicide was the cause of the twisting, the shoot also would be swollen, twisted or dying.

Herbicide injury is very difficult to correct. In most case you can’t correct the problem but this depends in part on the type of herbicide applied. In the case of eriophyid mites, a spray applied at the correct time, usually as leaves and needles swell in the spring, prevents further damage by this mite. Some eriophyid mites have multiple generations so more than one treatment of a miticide is required. As leaves and needles are damaged by mites, the reduction in photosynthetic tissue can result in plant stress. Stress invites other pests to attack the tree resulting in further damage and even death.

Dr. Curtis E. Swift is a retired horticulture agent with the Colorado State University Extension. Reach him at, 970-778-7866 or check out his blog at He owns Swift Horticultural Consulting and High Altitude Lavender.

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