Suspended Pavement at the Bartlett Tree Lab, Year 7

Here are this year’s photos from the Bartlett Tree Lab study comparing different planting methods for urban trees. For comparison, check out last year’s set. And for a fuller chronological picture, you can see all the photos up to that point in this post.

This independent study was set up by Dr. Tom Smiley to test how well trees fare in different growing media. The trees are monitored and tracked from year to year to measure their leaf extensions, height, DBH, and more.

(Structural Soil twig on the left, Suspended Pavement twig on the right).

(Structural Soil tree on the left, Suspended Pavement tree on the right).

The research plots were set up so that that 12 trees planted grown in a suspended pavement system (which is identical in performance to the Silva Cell), 12 trees were planted in a stalite/soil mix, 12 trees were planted in compacted soil, 12 trees in pure stalite, and 12 trees in a gravel/soil mix. All trees had access to 189 ft3 of growing medium.

It’s clear from the size of the trees and the color and number of leaves that the trees in the Suspended Pavement treatment are outperforming those in Structural Soil by a significant margin. Our expectation is that this advantage, which is so marked even at such an early stage, will only continue to compound over the course of their lives and development. We’ll continue to monitor their progress each year to monitor continuing changes.

Bartlett Tree Lab study is not sponsored by DeepRoot. Bartlett is an independent lab that chooses their own test parameters, conditions, and technologies. DeepRoot did not influence, pay for, or at any point become involved in the planning or design of that study. We also have no sway over the results, which we expect Tom Smiley to continue to publish about over the coming years.

All images courtesy of Tom Smiley.


  1. Gillis Dellebeke

    Can you give me a global composition of the used stalite, soil and gravel/soil, so I can compare them with Dutch (Europian) materials. Thanks for your help.

    • Leda Marritz

      Hi Gillis, I’ll see if I can get that information from Tom Smiley.

    • Leda Marritz

      Hi Gillis,

      According to Tom Smiley, treatments were as follows:
      1. Gravel/soil mixture—comprised of 80% gravel 2.5 to
      3.5 cm (1 to 1.4 in) diameter and 20% sandy clay loam
      soil. A hydrogel (Terrasorb, fine) was sprayed on the
      gravel before mixing with soil. Lifts were 20.3 cm (8.12
      in) thick and were compacted with an impact compactor
      to 95% Proctor.
      2. Stalite/soil mixture—comprised of 80% Stalite 2 to 3.5
      cm (0.8 to 1.4 in) diameter mixed with 20% sandy clay
      loam. Stalite was wetted before mixing with soil. Lifts
      were 30.5 cm (12.2 in) thick and compacted with a
      vibratory plate compactor to the manufacturer’s specifications.
      3. Stalite alone was installed in 30.5 cm (12.2 in) lifts and
      compacted with a vibratory plate compactor to the
      manufacturer’s specifications.
      4. Compacted soil—sandy clay loam was installed in 20.3
      cm (8.12 in) lifts and compacted with an impact compactor
      to 95% Proctor.
      5. Noncompacted soil with suspended pavement was installed
      in the existing sandy clay loam soil. Biobarrier
      was trenched 61 cm (24.4 in) deep to make 3 × 3 m (9.9
      × 9.9 ft) plots. The soil at this site was decompacted
      using a backhoe excavator after tree planting using the
      method proposed by Rolf (1994).

  2. Gillis Dellebeke

    Thanks a lot for your help and time.

  3. Michael DiTonto

    Great idea to manage stormwater runoff.

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