BRINGING BACK NATIVE PLANTS
SHEET COMPOSTING OVERPOWERS WEEDS IN RESTORATION PROJECT
Deep mulching with yard trimmings establishes native trees, shrubs and grasses on popular area around Florida’s Tampa Bay.
Margie Lynn Stratton, Allen Barker and James Ragsdale

Cockroach bay — a favorite spot for fishermen, canoeists and wildlife lovers — has been called Florida’s “Jewel of Tampa Bay.” For 40 years prior to 1991, the upland surrounding Cockroach Bay was farmed and mined for oyster shells. Now the upland area is part of a habitat restoration project that is bringing back native plant species to this environmentally sensitive, biologically active bay.

As part of the restoration, an innovative process of deep mulching with yard trimmings is being used by the St. Petersburg (FL) Sanitation Department and other regional agencies. In this low-cost, relatively simple process, mulch is spread in a layer about one-foot thick. Allowing for 50 percent compaction, approximately 2,550 cubic yards (cy) or around 550 tons/acre were applied. The mulch was used to suppress vigorous weed growth and give the reclamation plantings a two-year growth advantage against weed competition. The mulch was allowed to decompose in place (sheet composting), providing nutrients and organic matter to the soil. Deep mulching also suppresses weeds and stabilizes soil temperature and soil moisture, leading to establishment of native upland trees, shrubs and grasses.

The objectives of an innovative grant from the Florida Department of Environmental Protection for this restoration project included controlling weed growth; determining the survival, growth, and establishment of newly planted and mulched trees and shrubs; and determining if soil fertility, moisture levels, temperatures and organic matter content are increased by sheet composting of yard trimmings mulch.

The restoration project began taking shape in 1991, when 650 acres (150 acres of intertidal wetlands and 500 acres of tomato farmland and decommissioned shell mining pits surrounding Cockroach Bay) were purchased for $2.1 million by the Hillsborough County Environmental Lands and Acquisition Program. Historically, the site had been cleared of native upland vegetation. The soil on the site is Myakka fine sand with rapid surface permeability but slow internal drainage. Natural fertility is considered low.

A few test areas where oaks were planted randomly and mulched in 1997 showed that deep mulching controlled weeds in the restored areas. In the fall of 1998, shipments of yard trimmings from St. Petersburg to Cockroach Bay began, and a 41-acre, native-habitat upland project site was selected and designed for planting by the Southwest Florida Water Management District and Hillsborough County Parks Department. Farming had cleared the land of native vegetation, but because the farmland remained fallow, intense weed growth followed.

BRINGING IN THE “TROOPS”

A total of 1,045 truckloads of shredded yard trimmings, held for three to 21 days of storage in windrows, were required to cover the area with 12 inches of mulch. Approximately 22,720 tons or 118,000 cy of yard trimmings were applied between October, 1998 and April, 1999. The total cost was $6,329/acre and included land clearing at $266/acre; loading yard trimmings and shipping 32 miles at $3,656/acre; mulch spreading at $492/acre; plant purchases at $1,000/acre; and plant installation at $915/acre. The combined grant project expenses including in-kind contributions were $450,000. Total net savings to participating municipalities generating the yard trimmings were $192,500 in avoided disposal and reduced processing and handling costs.

In one 21-acre area, irrigation furrows were disked level before the entire area was covered with approximately 12 inches of yard trimmings (ground to minus four inches in length). An 11-person crew of field supervisors and Americorps volunteers planted —randomly on 40-foot centers — slash pine, live oak, southern red cedar, and southern magnolia trees, and randomly on eight to ten-foot centers, beauty-berry, saw palmetto, and gamagrass. Holes for planting were dug through the mulch and into the sand using a 12-inch diameter power auger. The mulch was formed into a bowl-shaped, water-collection depression.
For estimation of weed invasion, four 20 foot by 20 foot plots were marked and monitored monthly for coverage by weeds and invasive exotic plants from January, 1999 to May, 1999. Randomly chosen installed plants of each species were measured at planting and at 180 days after planting. Base caliper and height of trees and shrubs were measured as well as number of blades of gamagrass, saw palmetto fronds, and magnolia leaves. Soil or mulch samples were analyzed for plant nutrients, C:N ratio, organic matter, metals, pH, cation exchange capacity (CEC), organic matter (OM), and soluble salts (EC) at the beginning and end of the experiment.
WEED INVASION
Population of weeds was counted prior to mulching in areas undisturbed by disking. The weeds were covered with the mulch. The mulched plots after 140 days contained a lesser diversity and lower population of weeds than initially, but some species, such as white sweet clover and nutsedge, had grown, and some vines including the initially present Virginia creeper had spread. In stark contrast, the unmulched plot was fully covered with tall weeds of various species within 140 days. In areas in which mulch spreading was uneven and only about six inches deep, weed growth was stimulated by the mulch layer.
In the entire mulched acreage, white sweet clover grew in the northern part of the field and spread southward to cover about 1.5 acres. In zones where Brazilian pepper trees were present before the mulch was applied, seedlings emerged around the planting holes, as seeds were brought to the surface. Areas that had been disked to level the irrigation ditches had more weeds than areas where no disking occurred.
PLANT GROWTH AND SOIL MEASUREMENTS
Except for the beauty-berry and magnolia, plant survival was 70 to 100 percent of the population installed. The best survival was with the saw palmetto and gamagrass, which also grew well after planting. All surviving plants except beauty-berry grew after planting. Although beauty-berry exhibited shoot dieback, new leaf growth indicated refoliation and potential for renewed growth. Survival of magnolia was poor relative to the other trees, but surviving plants were growing as noted by a 100 percent increase in number of leaves. Survival numbers (between 70 to 100 percent) were high in most of the introduced species, especially compared to survival in reclaimed mine spoils (survival of less than ten percent).
Over a period of 140 days, soil pH, soluble salts and cation exchange capacity increased. These increases appeared to be associated with an increase in soil organic matter and available plant nutrients. Soil C:N ratio decreased from 17.4 to 15.7 as nitrogen concentration in the soil increased, whereas carbon did not increase. Estimated available nitrogen was 86 lb/A at the beginning and 108 lb/A at the end of the sampling period. Nitrate-N increased from 10 to 18 mg/kg; ammonium-N decreased from 2 to 0 mg/kg. All of these measurements indicate that soil fertility was improved by the mulch.
Fresh mulch was 26 percent water by mass, but after a three inch rain in March, the mulch contained 51 percent water and retained water at 47 percent total until the end of the experiment 60 days later. After this extensive rain, water remained in many of the planting holes for at least three weeks and may have contributed to the defoliation of beauty-berry and magnolia. Half of the beauty-berry and 25 percent of the magnolias defoliated. In addition, for about 30 days after the March rainfall, fungi flourished in the mulch (a yellow slime mold, a purple capped mushroom, and a tan, rosetted species). A fungus similar in appearance to Indian pipes also flourished. Throughout the experiment, after the heavy rain in March, the mulched soil remained at field capacity, about 17 percent water by mass. Except for the three weeks following the March rain, the unmulched soil remained dry in the top four inches.
Temperatures in the mulch were measured at four places in each plot and at four, eight, 12, and 16-inch depths. At 16 inches deep, the temperature remained about 80°F, despite the ambient temperature fluctuating from the 40° range to the 80° range. Temperatures in the mulched areas were higher than in the unmulched test spots for the duration of the experiment; however, conditions under high summer temperatures (90°F and higher) were not measured.
ANALYSIS OF MULCH
Over a period of 270 days, mulch pH increased, and soluble salts (EC) decreased. Moist bulk density increased. Organic matter and C:N ratio decreased. These data indicate decomposition of the mulch and overall leaching of salts. However, total extractable N increased from 0.68 to 0.9 percent, whereas nitrate increased from 3 to 18 mg N/kg, and ammonium decreased from 36 to 18 mg N/kg. Other extractable plant nutrients, except for Mn, increased in the mulch.
PROJECT RESULTS
Weeds and exotic invasive species were suppressed significantly by a 12-inch layer of mulch. Where mulch was spread to six inches or less due to uneven distribution, weed growth was stimulated, likely by the nutrients supplied by the mulch and by the insufficient weed-suppressing properties of the shallower depth. Weed germination also was stimulated perhaps by creation of a seedbed favorable for weed germination when disking of site soil to level irrigation ditches occurred before mulching. Plants installed in areas where Brazilian pepper trees had been felled while in seed had seedlings emerge in the soil surrounding their root balls. These issues can be solved in future undertakings. Disking should be avoided, and mulch should be spread uniformly at a depth of 12 inches after compaction. The mother Brazilian pepper trees could be removed before they go to seed. Hand weeding around each affected installed plant also should facilitate control.
Soil fertility was increased under the mulch. Soil temperature and moisture content were stabilized at about 80°F and 17.5 percent (field capacity). The warm moist soil and higher fertility contributed to the good establishment of the plantings. Continued decomposition of the mulch is expected to further increase soil fertility. During the experiment, the mulch decomposed and increased in extractable nutrients while decreasing in C:N ratio. By the end of the experiment, the mulch was stable compost at a C:N ratio of about 20 at which mineralization of nutrients should proceed readily.
Sheet composting of mulch is highly beneficial in reclaiming marginal soils. The benefits of common and exotic weed suppression, moisture and temperature stabilization, enhanced soil organic matter, and increased nutrient availability are valuable in this project and complement the financial savings in waste management. The removal of yard trimmings can now be seen as a recycling of nutrients and organic matter to improve and condition soils, rather than a disposal problem. This method has various and widespread applications, including control of exotic plants to restore native vegetation, construction projects, roadside plantings, reclamation projects, landfill closure plantings and other planting projects on marginal soils. m

Margie Lynn Stratton is with Manatee Community College, Bradentown, Florida; Allen Barker is with the University of Massachusetts, Amherst; James Ragsdale is mulch program coordinator with the St. Petersburg (Florida) Sanitation Department.