First off, some of you might be asking “what is a tree island?” Picture a tropical island surrounded with water, there is your island. Now place that island in your landscape surrounded by turf grass, plant a tree in it and, voila, you have a “tree island”. In landscape design, you might create the opposite effect, having the turf grass as an island surrounded by tree berms and other designated areas. Since our focus is on trees (that’s what arborists tend to do), we pretty much call anything that has separated trees from turf “tree islands”.
There are multiple purposes involved, but the two most common are:
1. Creativity in design – a way to make your landscape more interesting. (Personally, there is nothing more boring than seemingly endless acres of lawn with a lonely tree popping out here and there).
2. Separation of diverse plant species requiring different cultural practices, allowing more efficient management of each landscape component.
There is no doubt a tree island can set off a specimen plant(s) to advantage: drawing attention to unique structure, colorful foliage, showy blooms. The tree island may also be planted in a variety of compatible species for maximum effect and improvement of soil health. (A topic for another blog.) But an important feature is the ability to provide a buffer zone from management practices that are harmful to those plants within the island.
Tree islands also may provide diversity through compatible plant varieties.
Lawn care management practices can literally be deadly on trees. Weedwacking, the use of mechanical tools to mow grass and weeds away from the trunks, is more harmful than most people realize. They cannot conceive a little string can cause significant injury to a massive tree. Well, they can and do. And they can kill a young, thin-barked tree in an amazingly short period of time.
Damage is seen at the base of the tree where the bark is missing.
Lawn mowers also inflict damage. The repeated running into the tree causes permanent injuries. We were called out to remove an ash tree which was over 50 percent dead. The homeowners asked what we thought was killing the tree. One of the first things we do is look at the base of the trunk. Investigation revealed a significant injury encompassing almost 50% of the circumference of the tree. After we had made the final cut, I counted tree rings to see when the damage had begun. I told the homeowners the damage started approximately 11 years prior. The husband admitted that was about when he got his riding lawn mower and the flange of the mower hit the tree on that side almost every time he mowed.
Damage from flange on riding lawn mower.
Basal cut showing results of repetitive damage.
We have a tendency to think trees can withstand anything we do to them because we generally don’t see immediate consequences of our actions.
Other lawn care management practices which can harm trees and other plants are commonly used chemicals. Many products have “weed and feed” components. In addition to fertilizer an herbicide is included to deal with broadleaf weeds, such as dandelions. Trees are broadleaf species and some species are very sensitive to those chemicals. Tree islands at least back the herbicides off from the critical root zone. We recommend hand pulling or spot treatment of broadleaf weeds that cannot be tolerated rather than wholesale broadcasting of herbicides.
A short note on the roots of trees. They do not stop at the drip line. Many people feel you can do or apply anything if you stay outside the drip line. Well, the roots are out there. In fact, they can grow much further away from the trunk than the tree is tall. Tree roots grow wherever they can. They need space, oxygen and water. Some grow close to the surface, some can go much deeper. Many roots do congregate at the surface as that is where the most space, oxygen and water are and so are able to take up anything applied to the soil.
When designing a tree island, a common practice is to lay down weed fabric prior to applying some form of top dressing. This is not recommended when the top dressing is of an organic nature, such as wood chips, compost, or bark, which is what we recommend. Landscape fabric will block the biological exchange between the organic mulch and the mineral soil below. This means none of the benefits mulch can provide the soil will be available. This caution goes whether the fabric is pervious (allowing water and air to penetrate) or impervious (blocking water and air).
If you are using some kind of non-organic top dressing, such as rock, then it becomes less of an issue. I have personal prejudices against some of the non-organic products, but these will be dealt with in a future blog specific to top dressings.