We have 2 native and 1 invasive Najas species growing in Lake Arrowhead - Najas flexilis, Najas gracillima, and lastly Najas minor, detected in 2020. Small fragments can be difficult to tell apart, so I floated the 3 together in a tray to visually compare them:
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From left to right:
N. flexilis (slender naiad), N. minor (brittle naiad), N. gracillima (thread-like naiad) |
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Comparing naiad fruits and leaf bases
From left to right:
N. flexilis (slender naiad), N. minor (brittle naiad), N. gracillima (thread-like naiad) |
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Najas gracillima (on right) has very fine, thread like leaves that are minutely serrated, not always clear to the naked eye. It looks very grass like. Its leaves collapse in onto the stem when lifted out of the water, a helpful feature which Dale has observed in ruling out the invasive naiad. Interestingly, the fruits/seeds that grow in the leaf axils show up as little dense nodules along the stem, as obvious, solid intersections of leaf with stem. When compared with how the fruits appear on Najas minor, it's almost as if they are the wrong scale for the fineness of the leaf structure - they stand out.Najas minor (center) has stiff, crispy feeling leaves with stark and obvious serrations along their margin, that are immediately clear to the naked eye. The olive green leaves (in summer) have a clear recurving arch to them. They feel bristly to the touch. When lifted out of the water, the plant retains its shape and structure - it does not collapse in on itself. From above, these plants appear to be more substantial and solid than they actually are - a fully formed plant looks like a bunch of balloons from above, connected by thin, flimsy strings to the substrate. The plant easily fragments into sections from wave action, physical disturbance or late season maturity, carrying and distributing seeds throughout the water body.
We often find them growing in rocky, pebbly or sandy ground, sometimes as a monoculture (a hedge), and sometimes interspersed singly with other plants. They often grow alongside and amongst the native naiads.
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| A mix of naiads picked up as one handful |
We have seen fruits appear in the axils of leaves towards the end of July.
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| Corn on the cob shaped fruit in axil of N. minor leaf |
Najas flexilis has no visible serrations, though they are present, and are only seen under high magnification. The leaves arch, but not markedly so in a fixed recurve. The leaves are a lighter green than N. minor, and can feel somewhat stiff when old. When the plant bobs and floats in the current, there appear to be striations of color along the length of the stem, differentiating nodes and leaf axils from stems.
In midsummer (end of July) the stems of N. flexilis can have a metallic crimson color in Lake Arrowhead, which is quite striking.
Update 2022-09-13
When these plants are found in the water, they are very easy to tell apart as N. minor grows rapidly into a large, dense, bush. The leaves of the plant are buoyant and pull the plant up towards the water surface.
These 'bushes' are often 4 ft. tall and rise above the surrounding plants like a bunch of helium balloons on strings.
