Create a Forest for Your Trees

by Michael Kuhns, Extension Forestry Specialist (originally written for TreeUtah newsletter)

Want to grow healthy, beautiful trees where trees aren't normally found? Remember where they come from - the forest. Then create a forest for them to live in.

Nearly all the trees we grow in our urban landscapes are genetically adapted to living close together with other trees in a forest. But we often treat these trees as individual landscape elements with little regard to how they are affected by their surroundings or by other trees in the landscape.

The best way to acknowledge this need for trees to be in a forest is to create mini-forests. Forests are groups of trees and other plants, are shady with a few sunny openings, and have organic matter covering the ground instead of grass. These are conditions we need to recreate when we create a mini-forest. To create mini-forests:

• Determine where in your landscape you want your mini-forests to be so trees and related plants can have priority in those areas. These mini-forests must be large enough to contain several trees so the trees provide mutual protection and shade. You can group existing trees or plant new ones to create your groups.
• Mulch the ground around your mini-forests. Organic matter like wood chips, bark, compost, leaves, or even grass clippings can be used for mulch. These mulch areas should surround your tree groups and should extend at least several feet beyond all of the trees in a group. Coarse mulches can be four inches deep while finer mulches should be shallower.
• Avoid using plastic sheeting or weed barrier fabrics under the mulch. You don't see these in real forests -- they interfere with oxygen, water, and organic matter movement into the soil.
• Give trees top priority within your mini-forests. Don't manage for turf or sun-loving or annual plants in these areas. Small shade-tolerant trees and shrubs or herbaceous perennials can be planted under the larger trees but be careful to minimize tree root damage.
• Minimize herbicide use in your mini-forests to avoid tree damage.
• Only fertilize your mini-forests if there are signs of nutrient deficiencies, such as inadequate growth or poor vigor. Forests often are nutrient-poor, yet trees do well.
• Irrigate adequately to meet tree needs. You need to know enough about the particular trees you are growing to supply them with enough water while avoiding over-watering. Too much irrigation kills roots, wastes water, and can add carbonates to the soil that raise soil pH.
• Adjust mini-forest design and management to match needs of the specific trees involved. For example, junipers might do better grown at a wide spacing with little water, while sugar maples would do better with some closer grouping and will need more water. All plants in a group need to be compatible in terms of water needs, size, shade tolerance (depends on vertical position), and other features.

The look of a mini-forest landscape is one with clumps of overstory trees with associated understory shrubs and other plants scattered amongst patches of turf and other gardens. It's an attractive look that recognizes the needs of different types of plants and facilitates their management.