Nursery Engineering

Linda Benedict  |  5/16/2005 11:29:39 PM

Richard L. Parish

Raising plants in containers creates unique problems for nursery growers. A series of tests were undertaken to help assess the benefits of different growing techniques.

Plant Blow-over

Blow-over of plants is a major problem for container nurseries. This is especially true with container-grown trees because they are tall and targets for wind. Blow-over causes:

  • Increased labor costs setting the trees back upright
  • Damage to trees, reducing their value
  • Loss of granular fertilizer
  • Drying out of trees not watered properly while down

Nursery growers have tried several methods including guy wires on individual trees, trellis wires down the row and stakes driven through or beside containers. Also, some nursery supply companies have developed devices to stabilize plants and containers.

A test of systems to prevent or reduce blow-down of trees in 15-gallon containers included eight treatments, each with loblolly pine, bald cypress and Nutall oak. Treatments included the nursery standard of one steel rod (or stake) driven through the pot into the ground, two types of wire basket supports (two with and one without staking), plastic pot-in-pot supports (with and without staking), a trellis system with straps, and individual stakes (steel fence posts) with plastic supports.

Labor and material costs ranged from 33 cents per container for the standard steel rod to $11.29 per container for the trellis system. However, the components of most of the systems can be reused for several years with little or no additional labor cost.

The first problem noted in the 15-gallon test was partial collapse of the plastic pot-in-pot supports with and without stakes and with all three types of trees. Nevertheless, there was not enough damage to the inner containers to affect sale, and the outer containers were judged to be reusable.

A "reset ratio" is determined by dividing total trees blown over and reset during the season by the number of trees in the treatment. The higher the reset ratio, the higher the resulting labor cost. Considering the annualized cost of the various treatments and the cost of resetting as provided by the cooperating nursery, the annual cost per tree per treatment ranged from 12 cents for Nutall oak with the standard stake to $1.63 for bald cypress with a staked basket.

A second test at the same nursery used pond cypress
and live oak trees in four-gallon pots. One treatment used a horizontal rod on top of a 20-foot row of pots held down by steel hooks driven into the ground. The second treatment was the standard steel rod driven through each container. For both treatments, two outer rows were staked and three rows were not restrained. Data on number of blown-over trees were collected throughout the season.

Table 1 shows the reset ratios for the test of horizontal rods in four-gallon containers. The data show that the horizontal rod treatment was less effective than the standard vertical rod. The problem with the horizontal rods appeared to be an inadequate number of hooks holding the horizontal rods down. In the vertical rod treatment, each container had vertical rods whereas the horizontal rods were anchored with hooks every four containers. Under strong winds, the hooks pulled loose.

Granular Herbicide Application

Controlling weeds in the containers of ornamental plants often involves applying granular herbicides with a hand-held rotary broadcast spreader. Losses from such applications can be significant. This study compared losses from rotary broadcast, drop and band spreaders using three container spacing configurations and five species of plants.

Clay granules (a common pesticide carrier) were used
for this experiment. Plant species used were liriope, prostrate juniper, monkey grass, azalea and dwarf gardenia. These species represent a wide range of plant canopy openness. All of the plants were in one-gallon nursery containers.

Three container configurations were evaluated:

Containers were placed rim-to-rim in a hexagonal pattern with a straight line of containers in the direction of spreader travel.

Containers were placed rim-to-rim in a square pattern.

Containers were placed in a square pattern on 12-inch centers in each direction.

The first part of the study used a hand-cranked rotary applicator with two plant species – monkey grass and azalea. The containers were arranged in beds approximately 6 feet wide. Granules that missed the containers were caught on strips of polyethylene film under the containers and on test strips without containers. The spreader was operated on each side of the beds of plant containers. The test was replicated four times.

A special fixture built to test the drop-type spreader used an elevated wooden track over a wire mesh table on which the plants were placed. A sheet of polyethylene film on the floor under the wire mesh collected granular material that missed the containers. Each species of plant was placed on the wire mesh in each of the three configurations, and material that missed was collected and measured. Each test was replicated four times. After these trials were completed, the spreader was modified to disperse the material in bands over the containers. Only configuration 3 was used for this series of tests.

Losses with a rotary spreader – 37 percent with configuration 1 and 87 percent with configuration 3 – were higher than with a drop-type spreader – 13 percent with configuration 1 and 76 percent with configuration 3.

When the drop-type spreader was modified for banding, losses were reduced to a rate of 48 percent to 70 percent compared with 72 percent to 86 percent before modification. Plant species and container spacing configuration had significant effects on material loss. Drop spreader losses ranged from a low of 10 percent with closely spaced juniper to 86 percent with widely spaced liriope. With a rim-to-rim hexagonal configuration, the losses varied from 10 percent with juniper to 20 percent with liriope. With a rim-to-rim square configuration, the losses varied from 15 percent with azalea to 31 percent with liriope. With configuration 3, the losses were higher, ranging from 72 percent to 86 percent. Application efficiency was better with close-spaced plants.

Application efficiency improved with a more-controlled application pattern. Material was still lost between the containers in a line parallel to the direction of travel, but losses between containers perpendicular to the line of travel were greatly reduced in most cases.

This study demonstrates the extremely low application efficiency of rotary broadcast application on widely spaced plant containers. The losses in that case were as high as 87 percent of the granular material applied. Using tight container spacing reduced the losses with a rotary spreader to 37 percent. Using a drop-type spreader somewhat reduced the losses with widely spaced containers to a range of 72 percent to 86 percent and reduced the losses with tight container spacing to 10 percent to 20 percent. Modifying a drop spreader to apply discrete bands further reduced losses with the widely-spaced containers to 48 percent to 75 percent.

Efficacy of granular material application under typical nursery conditions can be quite poor. This study demonstrates that it is possible to significantly improve that efficiency by using different but commercially available application equipment. We suggest nurseries consider using drop-type spreaders rather than rotary spreaders for applying granular materials. Large, high-clearance drop spreaders are commercially available in widths up to 12 feet. Using such a spreader in a broadcast mode for closely spaced containers and modifying it to band on widely spaced containers would significantly improve the efficiency of granular herbicide application. This improvement in application efficiency would reduce costs and reduce environmental contamination from lost herbicide.
Wayne C. Porter, Area Extension Agent, Mississippi Cooperative Extension Service, and formerly at the Hammond Research Station, contributed to the granular herbicide application research.

Richard L. Parish, Professor, Hammond Research Station, Hammond, La.

(This article appeared in the spring 2005 issue of Louisiana Agriculture.)

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