Tree Planting Cohorts: Tracking Survival Beyond Year One

Most councils can tell you how many trees they planted last year. Fewer can tell you how many of those trees are still alive two, three or five years later. The gap between those two numbers — planted versus established — is the actual contribution to canopy.

If the urban forest strategy depends on growing canopy, planting numbers alone do not tell the story. Survival data does.

Why planting numbers overstate progress

A newly planted tree is not yet canopy. It is a commitment to canopy that may or may not be realised depending on establishment success. Failure rates for street tree planting vary widely depending on species, site preparation, watering regime, vandalism, drought and luck. Even well-run programs lose a meaningful share of plantings in the first year or two.

When canopy reporting counts planted trees as success, it overstates progress and obscures the part of the program — establishment — where the highest-leverage improvements usually live.

What survival tracking actually requires

Survival tracking is not a separate system. It is a small addition to the planting record:

• Planting date, species, location and supplier
• Cohort identifier (the planting program or season)
• Establishment check dates (typically 3, 12 and 24 months)
• Status at each check (alive and establishing, struggling, dead, replaced)
• Reason where status changes (drought, vandalism, root failure, unknown)

That data, captured consistently over a few years, lets the team report not just planting numbers but cohort survival curves. A cohort that loses 20% in year one and another 10% in year two is telling you something different from one that loses 5% across the same period.

Cohort-level patterns are what improve programs

Aggregate survival numbers are useful for reporting. Cohort-level patterns are what actually improve the program. They surface questions like:

• Are specific species failing more often, controlling for site?
• Is a particular planting season under-performing?
• Is one supplier's stock establishing better than another's?
• Are certain precincts losing trees at higher rates?
• Is the watering schedule reaching the high-loss precincts?

None of those questions can be answered from a single year's planting total. They emerge only when survival data is captured at the cohort level over time.

Why this often does not happen

The usual reason survival tracking does not exist is structural. The planting program is funded as a discrete project. Establishment care lives in a different budget. Survival assessment lives in nobody's budget. The planting record closes when the tree is in the ground. By the time anyone asks how many survived, the data was never captured.

The fix is not to start a separate establishment program. It is to extend the planting record to include the establishment checks, and to make those checks a named responsibility.

A minimum viable practice

A council that does not currently track survival can usually start with: a 12-month establishment check on every planting, a structured status at the check, and an annual report showing year-one survival by species and precinct. That is enough to start surfacing patterns. The 3-month and 24-month checks can come later.

Done routinely for a few years, this changes the conversation about canopy reporting. Instead of reporting planting events, the council can report established trees — which is what the canopy strategy is actually about.