Moonglow Juniper and picea abies pendula near sidewalks OK?

Asked December 13, 2018, 12:23 PM EST

Will either of these evergreens heave pavement? If so will pruning roots by inserting a spade at sidewalk edge during dormancy keep the roots in check? what is a safe distance for both for plant and pavement health? The catch all 30 feet tree distance sounds like overkill.

Queens County New York

3 Responses

For planting near a sidewalk you have to consider stability, adaptability to adverse conditions and height relative to any overhead wires as well as the challenge to the pavements.

Both plants should fall below the 30" foot height of standard overhead utility lines. The Picea abies 'Pendula' however can vary in stature considerably from plant to plant and typically needs to be staked and supported to grow well. Both plants will suffer in areas of compacted soil which are typically what would be encountered close to a paved area. The Juniperus scopulorum 'Moonglow', in particular, is severely affected by Cerospora needle blight and botryospaeria canker, which will be exacerbated by drainage issues.

Tree root systems are very difficult to study for clear reasons, so these rules of thumb for distance of planting get used. Conifers have shallow roots systems for the most part, but root spread is challenged by underground impediments, including compaction. They may spread unevenly when obstacle are encountered and create a stability risk. The shallow roots also affect their ability to withstand the salt water sprayed up from the road in winter.

Trunk diameter is the best method of estimating the need for root space but the calculation changes as the tree ages. As a substitute, allowing 2 - 3 times the drip line of the mature tree is considered fairly reliable. That would be 16 to 24 feet approximately for an average Moonglow juniper. The weeping spruce varies too greatly in mature form to estimate.

Do not cut off roots at the edge of the pavement or you risk destabilizing the tree. The tree also takes more than half its absorption from roots beyond the dripline so cutting the outer roots will come at a cost to plant health.

For a good discussion of trees that work well with the stress of an urban environment see this Cornell publication:

Thank you for the info, and the detailed PDF.

Both of these trees are located in a back yard by fences, and though next to pavement, they are in corners that will not be stepped in except for yard care. Walking through them merely leads to walls. And patio furniture placement in summer will make other traffic unlikely. So compaction (I hope) and salt are not serious issues.

I am not sure how to view the dripline of either tree. Can you advise if these additional facts will impact dripline?

The Moonglow Juniper, though capable of growing much wider, is often trimmed tightly for topiary, and we intend to keep it roughly 3 feet wide, and then maintain the height at around 8 ft through very scheduled pruning.

The Norway Spruce is already trained to bend at 5 ft high, and the leader is no longer flexible. We will train the bent leader, and lower branches it to grow horizontally in a narrow plane along the fence. So stability is not such a concern given the fence support structure.

My hope was the juniper will fare well in spite of the nearby pavement, since it is native to poor rocky soil with lots of big obstacles. And I hope stability wont be an issue, as it has wind breaks on 3 sides, and the small city garden itself has many wind breaks.

Maintaining a tighter canopy than would naturally occur may reduce root spread somewhat but does not guarantee it. Less leaf cover means less need for moisture from the ground but the plant's natural root growth will be encouraged by dry weather, the plant's need for stability and genetic habit. Obstacles or compaction in the root space (extending in all directions to 2-3 times the width of the natural canopy, not just directly under the tree), will encourage greater root growth in less restricted directions.