This inquiry raises a topic well worth discussion:
“… along the rear of our property line, there’s a group of six blue oaks in close proximity, which I’d like to extend with three or four oaks. A local nursery has some 15 feet tall. This will screen us from the rear neighbors.
Do you have any concerns about planting them approximately 10 feet apart, trunk-to-trunk?”
The desire for quick screening, or instant curb appeal in new landscapes comes up quite often. It might be to screen a new house next door or a new commercial property where the owner wants the look of established greenery. In many cases this results in crowded plantings, premature removal of crowded trees or other woody plants, or the need for additional, costly maintenance pruning.
For perspective, think in terms of nature’s way with crowded trees.
When you walk in a forest or oak woodland and look closely at the trees, you may notice some are big and full-crowned while others are smaller or over-shadowed, depending on the distance between trees.
In forestry lingo, these differences are termed “crown class.” Not all trees grow at the same rate. Even trees of the same species and age may grow at different rates. Slower-growing trees tend to lose out to faster growing trees, largely because of shade suppression.
The crown classes are:
Dominant: Trees with crowns growing above the height of neighboring trees.
Codominant: Trees forming the general height of the group or stand.
Intermediate: Trees with heights somewhat lower than the codominant trees.
Suppressed: Trees overtopped (shaded) by others nearby.
Now, apply that to a hedge or tree row planted for screening.
In a formal hedge of one species, each plant is pruned to the same height and width as all of the others. The plants are codominant.
Many of the plants used in hedges grow to tree-like proportions. Here in the Napa area common examples include Photinia fraseri, several species of Pittosporum, Podocarpus (now Afrocarpus) gracilior, Carolina cherry laurel, and Grecian sweet bay laurel (Laurus nobilis).
The lower branches tend to become suppressed, decline and die in the shade of upper branches unless the gardener intervenes. Continual pruning of upper branches helps to maintain the screening effect of foliage on lower branches by allowing sunlight to reach them. And the tip pruning on individual branches induces lower buds and lateral branch tips to grow.
Large-growing tree species can be maintained as relatively small hedges with consistent pruning. Examples include coast redwood and coast live oak. But the pruning to maintain such massive trees as small hedges is an intensive, long-term commitment of time and energy.
A hedge-row of trees planted for screening is similar to a hedge, but on a larger scale.
The blue oaks, mentioned above are not good candidates for pruning as a sheared hedge, and they are not evergreen, but they will form a good screen. The potential mature height and spread of a blue oak (Quercus douglasii) is up to 50 feet tall and 70 feet wide, so how can it make sense to plant them 10 feet apart?
Left to their own devices, some will tend to become dominant, others codominant, intermediate, or suppressed but overall, they will provide the desired screening. For a more uniform appearance, dominant trees can be pruned for extra thinning to allow more light and space for trees that are becoming “intermediate” or “suppressed.”
The species is relatively slow growing, so the pruning can be done at a fairly leisurely pace over many years. Good tree care is a long-term, low-intensity process.