by Dale Schultz and Debbie Broderick
A technique is described for removing N.minor using low tech methods with minimal spread of fragments.
Background
Najas minor is an annual aquatic plant that becomes extremely brittle in the fall. The plant disintegrates very easily and this is thought to be the mechanism for its seed distribution, as the seeds are abundant throughout the plant, located in the leaf nodes. Removal of the plant therefore demands great care to prevent fragments of the plant from being allowed to escape during the removal process.
Diver assisted suction harvesting has been used for the removal of N.minor, but this approach is expensive as it requires special equipment, three people, and special certification. This makes it unsuitable for removing single plants that are scattered over a wide area. It is more suited for large dense growths that cannot be handled by the technique described here.
Video
Video of the technique can be viewed at: https://youtu.be/wCjYW4SpJdg
There is also a rough cut of some video put together by Steve Underwood on 2020-10-01: https://youtu.be/osC07rMBC00
Equipment
Mask with purge snorkel
Landing net with strong handle, ideally 5 ft. long
Fine mesh (⅛” diameter holes or smaller)
Hoop size: 18” diameter
One side clearly marked along length of handle (spray paint)
Temporary marker (as described here)
Kayak
Dive flag
Foot covering (to protect against fish hooks and such hazards)
Plastic bin (small enough to fit between legs of kayaker) lined with a plastic trash bag to empty plants into
Small hand net for kayaker to use.
Optional equipment
Wetsuit & gloves (highly recommended after mid September in Maine)
Snorkel dive flag
Equipment not to be used
Flippers/fins. They cause too much disturbance and will cause fragmentation.
Technique
The operation is designed to be carried out by two people with one person in a kayak and one person in the water with a snorkel.
Individual plants
Select a N.minor plant to be removed. It should be a single plant, small enough to fit completely within the landing net.
The snorkeler should remain at the surface in a horizontal position to ensure that silt is not stirred up and the N.minor is not subjected to any water wash.
If not already marked, mark the plant location so that it can be revisited later, to check for plant remnants.
Obtain the landing net from the kayaker. Use the marked side of the handle to ensure that the net always goes in one direction through the water and does not get turned inside out. This prevents fragments remaining inside the net from being released.
The snorkeler should float on the surface and look all around the plant to see if there are other plants nearby and determine a ‘sweep path’ for the net.
Sweep the landing net through the water so that it inflates fully and then bring it down slowly over the plant from above. (Pretend the plant is asleep, and you don’t want to wake it up.)
When the net reaches the bottom, twist the net to move it sideways so that it closes over the plant, and pull the net up (slowly) so that the stalk of the plant is broken. Don’t worry about the roots. Najas minor is an annual plant, and all roots will die at the end of the growing season.
Push any surrounding plant material that is half caught up, into the net. Do not try to pull out other species of plants.
Look for fragments of N.minor that may now be drifting free. Capture all fragments (any species) that can be seen by keeping a forward motion on the net. The captured plants and fragments will stay in the net so long as one does not reverse its direction. Do not panic at seeing fragments and try to make fast sweeps. Sweep slowly, they don't rush off in lake water.
When no more fragments can be seen, move to the next plant if there is room in the net or return the net to the kayaker to be emptied.
The kayaker watches the surface and scoops up any fragments that may have escaped with a small hand net.
Kayaker must ensure that the net is not turned inside out during the emptying process.
Once any stirred up debris and silt has cleared:
Revisit each location and check for any remaining remnants of the removed plant.
If remnants are present, repeat the removal process.
If no plant remnants are present, signal to the kayaker that the marker can be lifted and stored.
Large plants that do not fit into a landing net
The technique is similar to that used for single plants but one needs to use a large Baleen net instead of the landing net.
The baleen net also works best with plants that fit into it completely. If the clump of plants is even bigger than the baleen net, then the landing net seems to be more suitable for carving off chunks, because the fragments that are produced can immediately be swept up more easily with the landing net. Switching from the baleen to the hand net delays sweeping if there is only one person in the water.
Construction of Baleen net
Materials
15’ of ¾” PEX piping
¾” wooden dowel
20’ of mosquito netting about 54” wide. (We bought ours at REI decades ago.)
Spray paint (different color to the tubing)
Tools
Something to cut pex, (hacksaw or PEX pipe cutter)
Sewing machine
Optional: Sanding machine to reduce diameter of dowels
Method
Cut two dowel pieces about 3” long and sand them down so they fit tightly inside the ¾” PEX tubing
Cut and join pieces of PEX tubing so that you have a piece about 15’ long. (They come in 10’ lengths so you will likely need two joins.)
Bend the pipe into a hoop and join the two ends.
Lay the mosquito net out on the floor and roll the circle of PEX pipe along it to determine the exact circumference of your hoop.
Spray paint one side of the hoop so that it is clear which side is the ‘Naiad’ side.
Cut the netting about 2” longer than the mark
Fold 4” of netting over on a long edge and sew it to make a hem large enough to take the PEX pipe
.Take the remaining netting and fold it in half and sew up the two sides forming a bag.
Thread a needle down the open end of the hemmed piece and gather it to match the size of the folded part, and sew the gathered edge on.
Thread the pipe through the hem and push the ends together.
Tips
If a N.minor is not growing upwards so that the entire plant can be captured from above, it is also possible to scoop it from below by bringing the hoop of the net to its base and then lifting the net up through the plant. It breaks easily and is swept into the net.
Take time to swim about and inspect the area around each plant. It is very important to know where other plants are so that you don't bump into them when sweeping or moving to the kayak. Be alert to drifting into plants when at the kayak. Move well away.
Slow movements work best. The best way to describe the approach is to regard the plants as sleeping, don’t disturb them until they are in the bag! To avoid dispersing any fragments, once a plant has been bagged, don't make swift net movements. Slow gentle sweeps work best.
When sweeping for fragments it is best to hold the net pole at the end opposite the net. This takes the net further way and provides a wider visual field for seeing fragments.
Avoid scooping the silt from the bottom, so that you are not dealing with a cloud of silt when sweeping for fragments. There is no need to get the roots out anyway.
When swimming with the landing net, it is easier to prop the pole up over one’s shoulder so that the net iself is above the water. Don’t lose track of which side of the net is the side dirty with fragments when returning it to the water.
Timing
We found that in Lake Arrowhead in 2020 the plants started falling over in early October. This suggests that one should aim to complete removals by the end of September. August may be optimal as the water is still swimmable (in southern Maine at least) and the plants should be easily found by late summer.
Discussion
Plants that can be captured from above can be removed without any fragments escaping.
Optimal conditions are when there is good sunlight and the water is clear.
The kayaker also provides a level of safety by having a large visible presence on the water surface.
The fine mesh of the net means that sweeping it through the water requires some strength, especially when using the net at its furthest extension. For this reason the kayaker needs to give the snorkeler room to twist about without bumping into the kayak. If there is a breeze, staying downwind reduces the chance of drifting onto the removal site.
If tackling growths that do not fit within the net being used, some fragments might get away. Having additional kayakers in the area skimming fragments is recommended.
After removal from the water, it is amazing how little true volume the plant represents!