In 2023, swollen bladderwort was added to Maine’s list of unwanted aquatic invasive plants, making it illegal to import, sell, and transport it under state law. Not long after, it was discovered in Tilton Pond (Fayette) by a volunteer plant patroller. Although this species of bladderwort is not native to Maine and poses a threat, there are nine native species of bladderwort in Maine. How do you know if what you are looking at is native or invasive? There are a few distinguishing characteristics to look for.
Of the nine native bladderworts here in the state, two are most commonly mistaken for the invasive: floating bladderwort and common bladderwort.
Floating bladderwort, when flowering, can look very similar to the invasive swollen bladderwort. They each produce the same circular “wagon wheel” floats to support their snapdragon-like yellow flowers. However, the size of the native species’ floats is significantly smaller than the invasive (half or quarter the size!). Floating bladderwort’s underwater leaves are much more delicate, opposed to the robust ones of swollen bladderwort. While the majority of swollen bladderwort plants typically flower in late spring (May to early June), floating bladderwort typically flowers later in the season (mid-July and August). You can see lots of these flowers now! It’s important to note that each may be seen in flowers outside of the noted timeframes; flowering tendencies vary between waterbodies and years.
Common bladderwort does not produce the aforementioned wagon wheel-like floats, only snapdragon-like yellow flowers, making it easier to identify when in bloom. However, the leaves are more robust, similar to the invasive swollen bladderwort. One key difference lies in the arrangement of the bladders on each leaf. On common bladderwort, the large bladders appear in an orderly, parallel pattern along the leaves, while on the invasive swollen bladderwort, all bladders appear randomly scattered, with no clear pattern on the leaf (think of sprinkling pepper). For more on bladder arrangement and identifying bladderworts when not in flower, including a key and images, click here.
Photo: Native floating bladderwort (left) next to the much larger invasive swollen bladderwort. Both have wagon wheel-like floats and yellow flowers.
This article was included in 30 Mile’s August 2025 newsletter. To view the full newsletter, click here.