We’re lucky in the Southeast. We have more trees in our towns and landscapes than most other parts of the United States. It probably goes back to the time when air-conditioning wasn’t even thought of yet and comfort, let alone just breathing, during the summer was dependent on having large shade trees.
This situation does have drawbacks. Arborist bills for instance. Having someone take care of that branch 40 feet up is pricey. Then there are all those roots, the shade, and fall leaves complicating lawn maintenance. If that sounds strange to you, hang around the extension office sometime and count the calls about thin grass and surface tree roots. Dense shade also changes the species of plant life usable in the parts of a landscape under trees. Often, nursery stock, particularly from the big-box stores, hasn’t been selected for shade and tree root competition.
That said, nearly everyone wants a big old tree. New ones are fine and dandy and full of promise, but it’s the large and aged that we enjoy most. These trees give us a sense of history, anchoring our homes and towns to a place in time and memory. Large trees are also amazing providers – from actual monetary value to physical, mental, and social health. The list of benefits, mainly from mature trees, is long and well researched. If you are unaware of just how important trees are and how well documented it is, try out Dr. Kathleen L. Wolf’s wonderful collection of work at The Human Dimensions of Urban Forestry and Urban Greening: thenaturewithin.info, it’s a great place to learn more.
There are some important differences between young and old trees that any gardener needs to know. It’s impossible to overstate how easy it is to fatally damage a big tree. Another interesting thing is that a big tree won’t show you that it has begun to die for years. The problems usually show up long after the action that pushed the tree over the edge, leaving you shaking your head as you write a big check to an arborist.
Massive old tree in lot obviously filling all the space.
Trees’ lives are complicated by 4 important things
Trees strive for balance, always. The balance we’re talking about is one in which resources are balanced between growth and maintenance. Growth and health of trees depends on resources being taken up by roots (mostly water), sugar being made by leaves, and pathways through the tree’s system for it all to be moved around. Old trees, even in a good situation, generally grow slowly. The cause of the slowdown is how much more tree the trees’ systems have to maintain and defend. Large, older trees have usually occupied all of their environment that they are able to. Their roots are spread out far and wide and the canopy is extended to its maximum. Removing branches or roots, or otherwise damaging the tree’s system will result in a loss a large older tree can’t balance out. Which brings us to problem number 2.
Most of the tree is already dead.
The interior parts of a tree are mostly dead and often aren’t even functioning other than as a sort of support pole and dumping ground for compounds the tree makes that it doesn’t want near the living parts. A big beautiful tree is actually a thin layer of living tree wrapped around its mostly dead interior. That mostly dead interior is also something the tree must vigorously defend. The bark over everything is generally that first defense. Once the outer layer of the tree is opened up, whether by pruning cuts, storms, or just accidents, that interior is now open to whole realms of nature that really want a chance at that interior. Since there is no life without injury, the tree develops defense systems to handle invasion by the outside world. Which brings us to the next big fact about trees.
Healing wound in trunk.
Trees can’t heal.
Not the way we do. An arborist will often say, “Trees don’t heal, they seal.” An injury to a tree is forever. After all, the tree is mostly dead with a dressing of living tree over it. Breaking through the bark leaves only the living layers around the edges of the wound to deal with injury. Trees typically do this by walling off the wound by plugging the cells with toxic compounds and then growing over the wound from the edges. The damage remains, hopefully sealed off forever. It is extremely common for older trees to be coping with numerous wounds and hollows caused by pruning, abrasion, tears, and breaks both above and below ground. Maintaining these compartmentalized pockets take up resources, further slowing growth. The extra resources needed to handle a new problem could be the resources the tree needs to grow enough to live. Which leads us to the fourth issue.
Trees have to grow.
It’s growth or death. Every year, a new ring on the trunk, new leaves, new flowers, new fruit, new stems, new bark, new roots. To stop growing is to die. Even when trees are obviously dying, growth is taking place wherever the tree can make it happen.
Large live oak shading lawn and home.
So living with your big old tree means keeping its needs in mind. Think of it as a really wonderful older pet out in your landscape. You don’t purposely injure a pet. You wouldn’t let just anyone cut it. You wouldn’t take away things it needs to live. We often make exceptions and rearrange our lives for our dogs and cats, why not our trees?
Garden With Big Trees
Trenching in lawn near a tree.
First, Do No Harm
The number-one killer of trees of any age is people – usually the very people who care about them. There are two kinds of damage that nearly every gardener does to their tree. The first is mower and string trimmer damage. Constant damage to trunks and structural roots by lawn equipment create more injury for the tree to wall off and maintain. For big trees, damaging structural roots could lead to destabilization.
The second is root cutting. Removal of tree roots is particularly devastating to trees. All trees need a constant flow of water from their root system to live. Removing a root, say an inch in diameter or larger (or just cutting through one) effectively removes miles of root system from the tree. This has the effect of putting the tree into a drought, even if it’s raining. The tree is now unbalanced. It has big water demands and now no way to satisfy them. Decline begins. Also, damage to large roots can allow decay to creep in. Many a hollow tree became so because of root damage.
Once a tree is mature, big structural changes to its canopy are usually not needed or recommended except when our lives and property are threatened by the tree. Pruning of large and older trees should be limited to crown cleaning, which is the removal of dead or damaged limbs from the canopy. If you fear for the structural integrity of an older tree’s canopy, discuss cabling and bracing with an ISA certified arborist.
Kill Some Grass
The effort many gardeners put into growing grass under the canopies of trees is amazing. If you garden WITH your older tree, remove the competition by killing off grass and mulching. This mimics the natural environments that trees evolved in, forests. Your tree will do much better and you won’t be slaving over and spending so much on turf.
Big old declining tree with dead branches
Embrace The Shade
Too often, we try to raise plants that can’t get along with trees, especially in the shade under the canopy. Read up on woodland gardening and select shrubs and perennials and even some annuals that get on well in the understory of forests.
Don’t Fertilize Your Tree To Death
Fertilizer is not the answer to everything, particularly trees. For older trees the problem is giving them lots of nitrogen, which boosts stem and foliar growth. This new burst of growth can actually create demand for water that the root system may not have access to. Be sure to have your soil tested and have your tree’s condition assessed by an ISA certified arborist before spreading lots of fertilizer around. If you want to fertilize plantings around an older tree, try the trick of foliar fertilization. Use one-eighth-strength liquid fertilizer and spray it onto the foliage of the plants you want to receive the extra nutrition. This will save you time, spare the tree, and save some money on fertilizer too.
A version of this article appeared in an April 2016 print edition of State-by-State Gardening.
Photography courtesy of William J. Rowe II.