Why Planting Trees in a Narrow Side Yard is a Mistake (And What to Do Instead)

The Dilemma

A homeowner recently asked:

My neighbor's new second-story ADU looks right into my yard and bedroom window. I have a narrow 5-foot gravel strip along the fence—what trees can I plant there to block their view without blocking my walkway?

The GardenOwl Diagnosis

The Scenario

It is the modern homeowner's nightmare: You wake up one morning to find a second-story ADU (Accessory Dwelling Unit) rising above your fence line. Suddenly, your private backyard and even your bedroom window are on display to your neighbor’s tenants.

Our homeowner in the Pacific Northwest is facing exactly this. They have a privacy crisis and a specific constraint: a narrow side yard that is only 5.42 feet wide between the wooden fence and their home's siding. Their instinct is to plant a row of tall privacy trees in that gravel strip to block the view.

This is a textbook case of The Boundary Displacement Syndrome. This pathology occurs when macro-scale flora (like trees) are forced into the narrow setback of a structural boundary. It feels like the logical solution—"I need height, so I need a tree"—but in a corridor this tight, it is a recipe for structural damage and maintenance misery.

The Trap: Why "Privacy Trees" Fail in Narrow Strips

That 5.42-foot measurement is a trap. In landscape architecture, we don't just look at the trunk size; we look at the canopy radius and the critical root zone.

If you plant a standard privacy tree—like an Arborvitae, Cypress, or even a columnar Holly—in a five-foot gap, you are creating three expensive problems:

1. The Humidity Trap: As the tree grows, it will press against your siding. This lack of airflow prevents your siding from drying out, leading to green algae, mold, and eventually rot.
2. The Walkway Block: Even narrow trees need a 3-to-4-foot radius to breathe. In a 5-foot space, the tree will consume the entire walkway within three years. You will be fighting through wet branches just to take out the trash.
3. Foundation Conflict: To support a 15-foot tree capable of blocking a second-story window, the root system needs to spread. In this corridor, the roots have nowhere to go but into your foundation drain or under the fence, eventually heaving the posts.

The Solution: Soft Engineering and Geometry

To fix The Boundary Displacement Syndrome, we stop thinking about "planting trees" and start thinking about "vertical architecture".

1. The Side Yard: Build Up, Not Out

In that tight 5-foot alley, you need height without width. The solution is a trellis system or fence-topper.

• The Structure: Install fence post extenders or bolt a cattle-panel trellis system directly to the existing fence posts. This gives you rigid structural support up to 8 or 10 feet without digging new holes in that narrow gravel path.
• The Plant: Since the homeowner is in the PNW, the perfect candidate is Clematis armandii (Evergreen Clematis). Unlike deciduous vines that leave you exposed in winter, Armandii keeps its thick, leathery leaves year-round. It grows rapidly to 20+ feet but only occupies about 6 inches of horizontal width. It creates a "green wall" that blocks the view without eating your walkway.

2. The Backyard: Use Geometry, Not Brute Force

For the wider backyard, avoid the Monoculture Screen Syndrome. Don't line trees up against the back fence like soldiers.

Instead, use the geometry of sightlines. If you stand on your patio and look at the neighbor's window, a tree planted closer to you (near the patio) needs to be much smaller to block the view than a tree planted 50 feet away at the fence line.

Plant a cluster of Pacific Wax Myrtle (Morella californica) or 'Excelsa' Cedar closer to your living area. This creates an immediate visual block for you, while leaving the fence line open for light and airflow. This layering effect makes the yard look larger, whereas a wall of Arborvitae at the fence makes the yard feel like a box.

The Diagnostic and Visualizing Safety Net

Before you drop hundreds of dollars on nursery stock, you need to verify your clearances. It is easy to imagine a tree fitting in a space, but seeing it in 3D reveals the claustrophobia instantly.

This is where a tool like GardenDream acts as your safety net. You can upload a photo of your narrow side yard and overlay a trellis versus a hedge. You will immediately see that the hedge makes the path unusable, while the trellis leaves the space open. Use the tool to "walk" the path virtually before you commit to a layout that might block your access.