Considerations

Water uses and functions

• No water use restrictions
• May be only option where herbicides are not effective or immediate removal is required
• Fast moving water where herbicide concentrations cannot be maintained
• In areas where herbicides cannot be applied (drinking water intakes, brackish water)
• For plants that are not susceptible to herbicide control (lyngbya)
• Remove masses of plants or organics from bridge pilings and flood control structures
• Removing floating (especially drifting) masses of organic material (peat, muck)

Fish and wildlife

• Coordinate activities around nesting and foraging activities, especially birds and alligators
• Non-selective – removes all plant and animal species that cannot escape harvester path
• May select for invasive plants
• Removes slow growing native plants mixed with invasive plants like hydrilla
• Hydrilla recovers faster from harvesting than native plants and shades them out
• FL study: up to 32% removal of young-of-year sport fish per harvest in hydrilla
• Studies in other states show similar results of harvesting fish and invertebrates

Control feasibility

• Harvest rate is generally slow
• Removes 2-8 acres of aquatic plants per day – removal rate is influenced by:
  • Plant type
    • Submersed plants more difficult to harvest than floating or emergent
    • Cutting blade and belt are well below water surface
    • Submersed plants are more difficult to see
    • Tall emergent plants like cattail are difficult to collect on conveyor
    • Woody species like willow are difficult to cut
  • Weight – aquatic plants are mostly water
    • Hydrilla weighs approximately 20 tons per acre; water hyacinth ranges from 200-800 tons per acre
    • Organic content – floating islands can be composed of peat up to four feet thick
• Invasive water hyacinth and hydrilla expand faster than harvesters can remove them
• Not effective in shallow water or in flooded timber reservoirs
• Cannot reach plants where control may be important (shorelines, among trees or snags)
• Fragments of invasive plants like water hyacinth are left for immediate recolonization
• Can spread invasive plants like hydrilla or milfoil by creating numerous viable fragments
• Turbidity issues possible in shallow water or when hauling muck or peat across lakes and rivers

Other considerations

• Expenses are generally high
  • Acquisition cost of machinery
  • High maintenance
  • Operating costs
    • Invasive plants like hydrilla may require 2-3 cuts per year
    • Costs can double for each mile material is hauled within a water body
    • In-lake hauling costs can be reduced using higher speed transport barges
    • In-lake disposal sites close to harvest area reduce project cost
    • Sacrifice small area of lake bottom to stack material from large harvest area
    • Large in-lake disposal sites may provide wildlife habitat
    • May be public opposition (unsightly; odor from decaying plants)
    • Costs escalate if harvested material must be hauled to external disposal site
    • Loading and trucking expenses
    • Sealed dump trucks may be required for over-the-road hauling
    • Disposal fees may be required (rent private land, landfill)
    • Disposal site preparation and restoration costs may include:
    • leveling, water containment, spreading, drying, chipping or burning woody material, temporary road preparation and dismantling, reseeding, pave streets or driveways damaged from heavy equipment
• Few uses for harvested material
  • Aquatic plants are mostly water (90%+)
  • Harvesting and processing costs exceed market value of any product
  • Organic material from floating islands can augment poor soil or be used as fill
• Cost-effective for small scale projects, especially native plant removal

Examples of Feasible Control:

• Harvest lyngbya from Crystal River where fast flowing springs and tidal action preclude herbicide use
• Remove floating islands of herbaceous and woody vegetation and buoyant organic sediments up to four feet thick on Lake Lafayette and Orange Lake
• Immediate removal of floating masses of plants from bridge pilings on the St Johns River
• Maintain established trail system through multiple native plant species on Lake Tsala Apopka where several different herbicides would be required to control different plant types.

Barge-Mounted Track Hoes or Draglines

Similar to harvesters except that material is lifted out of the water by grappling devices or buckets rather than conveyor belt. Applications and considerations are similar.

Considerations

• Water uses and functions

• Fish and wildlife

• Control feasibility

• Other considerations

  • Generally more cost-effective at removing floating islands than herbaceous plants
  • Pick up woody species or trees that could jam conveyor harvesters
  • Not cost-effective for submersed plant removal

Examples of Feasible Control:

• Barge-mounted dragline and hoes removed about 35 acres of dense floating island material (peat up to 4 feet thick, trees up to 10 inches in diameter and 50 feet tall) from boat ramps and residential shorelines and docks on Lake Pierce generated by Hurricane Charlie in 2004.