Winter Plant Adaptation, Part 1: Best Buds

Winter time blogging brings out my inner botany nerd. Species of the plant kingdom do some pretty wild and crazy things in their effort to survive (something I didn’t learn until my college days), so I shall describe some cool ways winter plants adapt via the following garden blog trilogy (I have always wanted to write a treeology).

One question I frequently receive this time of year centers on concerns of plant health through the extremes of winter weather, particularly bud survivability. I usually tell people not to worry about the dead of winter. Leaf buds are the hardiest portion of temperate plant species, with stems and leaves much more likely to dessicate.  It is late winter or early spring conditions for which I would have more concern.

As the days get shorter, plants prepare to enter winter dormancy. Dormancy is a plant adaptation that solves a number of biological and environmental problems. Plants might “shut down” due to environmental or genetic causes. Plant dormancy is seen in growing tissue ranging from root and leaf meristem, seeds and other vegetative portions. Plant may go dormant in arid regions during the dry portion of the year. In northern Indiana, moisture levels are not the biggest limiting factors; day length and temperature drive leaf bud dormancy here. There is not enough time or good quality light in the winter day to make food to survive upon, and temperatures are low enough for water crystals to slice through plant tissue. An interesting observation to make regarding day length, or photoperiod, on plants in temperate environments is what happens to tropical house plants through the season. House plants, especially those placed outdoors during the normal growing season, behave in similar ways to their temperate counterparts. Tropical plants growing in their native habitat show a fairly constant rates of growth or activity (certainly not the extremes of temperate plants), but here they enter a sort of dormancy of their own. Anyone moving plants in and out as the calendar dictates, experiences the rapid growth of house plants in the spring. Then what happens once plants are brought back indoors? Plant growth slows down to almost zero, and many shed a portion of their leaves. Why? Because there is not enough day length to feed that much tissue.

So what plant factors contribute to the onset and breaking of leaf bud dormancy? Longer dark periods induce plants to produce abscisic acid, a plant hormone involved with leaf drop and bud dormancy. It inhibits plant growth. Bud break is overcome through the production of two other important plant hormones: cytokinins and gibberellins. What I find particularly fascinating is the fact that cytokinins are produced in the tips of roots (which, when dormant, do not produce concentrations high enough to overcome dormancy), and sap does not move upward to reach the buds. It is only with warmer temperatures that production resumes, sap flows back up the stem to tickle leaf buds awake, and one of the main causes of buds unfurling is manifest.

Magnolia flower buds seem to particularly concern people. If you visit Wellfield Botanic Gardens’ Spring Garden, you will find a number of magnolia species and cultivars. The large, luscious, hairy flower buds look vulnerable to weather extremes. Yet, if one were to open them up, one could feel the tough the bud scale. Leathery scales and a nice fur coat keep these buds snug until spring sunlight perks things up.

Josh Steffen, Horticulture and Facilities Manager