Faced with pruning an overgrown, messy shrub, the first question on many gardeners’ minds is, “Where do I start?” Like a doctor tending to a trauma victim, your patient may have dozens of problems needing attention, easily seeming overwhelming at first.
But like a doctor you wouldn’t tackle everything at once; you’d start with the most dangerous things, the things that will lead to more problems if ignored. In plants, these prime pruning targets are the “Three D’s”: the dead, diseased, and damaged wood.
Why “Dead, Damaged, and Diseased” Is So Important
Plants are constantly assaulted by bacteria, viruses, fungi, and insects, much as we are constantly exposed to disease. In humans and plants, living cells have ways to constantly fight off these intruders, and an armor of skin or bark blocks them from getting in in the first place.
Wounds let infection in, dead cells cannot fight off infection, and weak cells can’t fight as hard. Plants usually have natural ways to isolate or shed dead, damaged, and diseased tissue, but those processes can take years. In the meantime, invading disease can gain a foothold. Pruning them out faster gives your plant a boost.
Proper pruning first targets and removes the places that are access points for disease: the dead, damaged, and diseased tissue. Since this wood has to go, removing it first lets you step back afterwards to reassess before you take the next step in pruning.
Dead tissue is a part of a plant in which all the cells have died and will never come back to life again. “Dead” is not “dormant”: in winter, all the wood on a tree might look dead, but in a healthy tree most of it is actually in a hibernation-like protective state called dormancy. On dormant wood, buds and cambium tissue within the wood are completely alive, waiting for a chemical signal to start working again.
When the soft stems of perennials die they quickly dry and turn brown. When wood of trees and shrubs die, there are a few, often subtler signs, including the presence or absence of leaves out of season, a lack of buds at the nodes, hollow wood, and missing bark.
Damaged tissue is synonymous with partially dead tissue. The entire branch or limb is not dead; in fact the whole thing may still be fully leafy and otherwise functioning as a live unit. The trouble is that, internally or externally, there has been damage to the plant that will cause weakness and trouble down the road.
One common example is impact to a tree that penetrates the bark, such as from a vehicle collision or a mower or string trimmer. This creates a dead zone that the plant will try to heal over over time. If the damage is too deep, healing cannot cover it or cannot cover it strongly.
Another example is a stem that has been bent too far, permanent crushing and creasing the wood or flesh, if not snapping outright. All plants are made to flex and can bend somewhat, but too far and they won’t recover. Sap flow is interrupted and the plant is likely to slowly die back to the bend.
In both of these cases, the part of the plant past the damage can live on a long time, perhaps forever. But the key point is that the architecture supporting and feeding the plant has been interrupted, and there is risk. Whenever damage is beyond minor, it’s best to remove the hurt branch to let a stronger one take its place.
Disease comes to plants in many forms, living attackers such as bacteria, fungi, and viruses. You can also think of infestations by insects as “disease” in that they begin in one part of the plant and spread throughout, harming it as they reproduce.
For the most part, a plant will not recover from a disease that you notice. By the time you see the sign of an infection, chances are that waiting for the plant to fight it off on its own is not a good idea if you have a choice. The best action instead is to assess if the problem is in one part of the plant, and if it is, cut that part out before the problem can spread.
Unlike wood that is simply dead or broken, diseased wood contains a living inoculant that can still spread and re-infect the living plant even after you cut it off. These may be insect eggs in the wood, fungus spores, or bacteria that can leap through the air riding splashes of rain onto a new plant—I kid you not.
For these reasons, it is not enough to just cut off diseased wood—you must remove it from the site as trash or destroy it by burning. You should also disinfect tools that cut into diseased wood before using them to cut again into good wood.