Do you remember the functions of xylem and phloem in trees from elementary school science class? Me neither, so here is a refresher with some tips for gardeners about tree care and pruning.
Xylem’s function is to move water from the roots to the leaves, where it releases into the atmosphere through tiny openings in the leaves called stomata. Before that happens, the soil minerals and root-stored compounds like growth regulators and some sugars are distributed to the leaves.
Although it seems like wasting water, evapotranspiration lowers leaf temperatures, cooling them when needed in the heat of summer. It is also the only way to move the nutrients to where they are needed. According to Linda Chalker Scott, well-known garden myth buster, professor, and writer, using antitranspitants on leaves can have negative effects on the plant.
Phloem’s function is to distribute sugars and nutrients produced through photosynthesis in the leaves primarily, and then stored in places like woody roots, trunks, and branches. The tree builds up carbohydrate reserves in the fall and then redistributes them in the spring. Phloem cells move nutrients multidirectionally, unlike xylem, which moves the nutrients only up.
When a tree is actively producing new leaves and flowers/seeds, the phloem moves the nutrients toward these tissues. If the gardener removes leaf-bearing branches and stems while pruning, they create a two-step problem. First, the main source of photosynthesis (nutrient production) is now gone. Secondly, because of that, the tree has an increased demand for nutrients, which it robs from the remainder of the plant. Carbohydrates are removed from the remaining leaves and woody storage tissues to the detriment of the whole tree. The lesson: do not prune live tissue during the growing season. Wait until dormancy. Also, do not prune large sections at one time. Spread it out over several winters.
One more caution. Because most of the phloem is just beneath the bark, it can easily become damaged by lawn mowers, string trimmers, or anything banging into it.
The Edgar County Master Gardeners will answer your questions all year long. Call 217-465-8585 or email schiver@illinois.edu.
Illinois Extension leads public outreach for University of Illinois by translating research into action plans that allow Illinois families, businesses, and community leaders to solve problems, make informed decisions, and adapt to changes and opportunities. Illinois Extension is part of the University of Illinois Urbana-Champaign College of Agricultural, Consumer and Environmental Sciences.