Koa pasture scarification, silvopasture, and plantation

August 17, 2009 ~ Kukaiau Ranch

Koa forests can be restored to upland pastures in Hawaii either by soil scarification to stimulate germination of buried seeds or by planting. Cattle may be able to graze in open koa stands in silvopasture systems, but cattle eat the tops of young koa and may damage roots and introduce diseases even in older trees. Most koa plantations have been damaged by livestock and have very low potential timber yield. Here we demonstrate different methods of soil scarification and herbicide use to regenerate koa. We also investigate timber yield and quality from a nearby 33-year-old koa plantation.


University of Hawai‘i at Manoa
College of Tropical Agriculture and Human Resources

USDA Forest Service Institute of Pacific Islands Forestry

Kukaiau Ranch


USDA Natural Resource Conservation Service Conservation Innovation Grant,, Hawaii contact: Mike Whitt, Resource Conservationist, USDA NRCS (Honolulu), 808-541-2600 ext. 153,

Video Virtual Field Days

  • Establishing Silvopastures with Acacia koa: Video of scarification for koa establishment at Kukaiau Ranch
  • Thinning, Fertilization and Herbicide Trials to improve Koa Production: Video of thinning, fertilization, and herbicide trials to improve koa production in a young koa stand at Umikoa Ranch
  • Grass Suppression for Koa Regeneration: Dr. James Leary, UH-CTAHR, conducts herbicide trials at the Kona Hema Reserve to control grasses suppressing regrowth of koa seedlings ( Acacia koa) using glyphosate (Roundup®), fluazifop-p-butyl (Fusilade®), and imazapyr (Habitat®). Dr. Leary also demonstrated use of a hands-free spraying system and evaluates the impacts of imazapyr on other native species (ohia, Metrosideros polymorpha)
  • Herbicide Trials on kikuyu grass at Hawaii Volcanoes National Park: Dr. James Leary, CTAHR Invasive Weed Specialist, initiates herbicide field trials with staff from the National Park Service (NPS) on kikuyu grass ( Pennisetum clandestinum) at Volcano National Park. Herbicides tested were: glyphosate and imazapyr. Two types of hands-free spraying systems are shown. Dr. Leary visits the site again and reports back on how the herbicides performed.

Handouts from the field day

(all files in pdf format acrobat reader icon)

References for further reading on koa


Herbicide application for grass suppression for koa establishment

Grass suppression 01

Cattle were removed from this pasture in August 2008 and lines sprayed with imazapyr (applied at 1 qt/acre) in September 2008. Photos were taken February 2009

Grass Suppression 02

The herbicide was effective in the healthy kikuyu grass growing under the residual koa canopy in some spots

Grass Suppression 03

The herbicide was much less effective in open areas of dense kikuyu grass.

Grass Suppression 04

While there was some injury to the treated kikuyu grass, the single application of imazapyr was not enough to cause mortality. A second application of imazapyr plus glyphosate applied at 1 qt/acre was made in March 2009.

Grass Suppression 05

A few koa seedlings were observed germinating in the treated kikuyu grass.

Grass Suppression 06

Koa seedlings were observed in areas where a bulldozer had passed, indicating that there is still a viable seed bank in the area.


Soil scarification for koa establishment

[photos by J. B. Friday]

Soil scarification 01

Using a pasture aerator to scarify for koa regeneration. The weight on the rolling drums causes the spikes to sink into the soil.

Soil Scarification 02

The effect of the spiked pasture aerator in a mowed strip and a strip where herbicide had been applied.

Soil scarification 03

The disk used for scarification.

Soil scarification 04

The disk in action, turning over the sod.

Soil scarification 05

A strip where herbicide had been applied 3 months previously vs tall grass in an untreated area that had not been grazed for a year.

Soil scarification 06

Mowing to knock down the tall grass in advance of scarification. The field had not been grazed for a year. Under operation conditions, a field could be grazed immediately before cattle are excluded and the area is scarified.

Soil scarification 07

The disk turns over the sod and disturbs the site more than the aerator.


Experimental harvest of five koa trees in the 33-year-old “cornfield” plantation at Kukaiau Ranch

[all photos are by J. B. Friday except for photo 9 by Jian Wang.]

Kukaiau cornfield 01

The “cornfield” koa was planted in 1976 by the USDA Forest Service as a demonstration that koa reforestation could work in upland pastures on Mauna Kea. The trees originally were planted at 4 x 4 foot spacing and were never thinned. Today, probably half of the trees have died from overcrowding.

Kukaiau cornfield 02

About twelve trees in the plantation had potential as future crop trees. These were marked with a single blue ribbon. “Cull” trees are trees that would be removed in a thinning operation to allow the crop trees to grow. Four of these were marked with double blue ribbons and measured.

Kukaiau cornfield 03

Four “cull” trees and one “crop” tree were felled for determination of wood yield and quality. This cull tree was 15.1 inches dbh.

Kukaiau cornfield 04

Logs were measured and scaled; this log scaled out as 70 bf Scribner, not allowing for defect.

Kukaiau cornfield 05

Butt log of cull tree showing sapwood and heartwood. Only the heartwood is merchantable.

Kukaiau cornfield 06

Logs were milled on site and tally was kept of yield of individual logs.

Kukaiau cornfield 07

This cull tree yielded 42 bf net. The wood was light in color with little figure.

Kukaiau cornfield 08

Some curl developed in the branch forks.

Kukaiau cornfield 09

Twelve trees in the plantation had potential as crop trees. Tree 1 had a dbh of 20.2 inches and was harvested for determination of yield and wood quality. Photo by Jian Wang.

Kukaiau cornfield 10

Logs were milled on site on a Wood-mizer band saw.

Kukaiau cornfield 11

Decay resulting from injuries years previously ran through the logs causing a significant loss in yield.

Kukaiau cornfield 12

Crop tree 1 yielded 200 board feet. Lumber was light in color with little figure.

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