Aquatic Plants of Pennsylvania. Timothy A. Block
Egeria densa
a Based on surveys of 115 lakes between 2000 and 2007.
Prevention is far more effective than attempts to control an invasive plant after it has become established. Prevention can take the form of limiting access by boats from other areas that might carry seeds or plant fragments and educating lake users to prevent deliberate or accidental introductions of non-native plants. Another approach is to protect water quality; nutrient enrichment (eutrophication) will exacerbate problems with over-abundant vegetation.
Figure 1.15. An infestation of the non-native, invasive water-chestnut (Trapa natans) at a lake in Bucks County, PA.
Physical control efforts such as pulling, cutting, or raking may provide temporary relief; however, the ability of many aquatic plants to propagate themselves from detached fragments should be kept in mind.
Over-abundant native species—Not all problems arising from excessive growth of aquatic vegetation are caused by non-native species. Under favorable growing conditions some native species can also form dense stands that interfere with recreational uses of lakes. Shallow water and high nutrient availability are most often the causes.
Opportunistic species that have created problems include fragrant waterlily (Nymphaea odorata) (Figure 1.16), Farwell’s water-milfoil (Myriophyllum farwellii), common water-milfoil (M. humile), broad-leaved water-milfoil (M. heterophyllum), waterweed (Elodea nuttallii), purple bladderwort (Utricularia purpurea), inflated bladderwort (Utricularia inflata), and coontail (Ceratophyllum demersum).
Prevention of problems caused by explosive growth of native species should focus on reducing nutrient inputs.
Figure 1.16. Water surface nearly covered by native fragrant water-lily (Nymphaea odorata) at a shallow pond in Lackawanna County, PA.
Chapter 2. Identification Keys for the Aquatic Plants of Pennsylvania
The keys in this book are designed to guide the user through a series of decisions leading to proper identification. At each step of the key, the user must examine a characteristic or characteristics of the plant and decide which of a pair of corresponding statements best fits the specimen in question. Each statement and its corresponding member are preceded by the same capital letter(s). There are a few simple rules that, if followed carefully, should ease the task of arriving at a correct identification.
1. Read each statement of a pair carefully and thoroughly before deciding which best fits the plant you are looking at.
2. Be sure you understand any botanical terminology used in the statements before moving on to the next step. Consult the glossary at the back of the book if you are not certain.
3. Look at more than one example of the characteristic in question. For example, if the key asks about the number of petals present in a flower, look at more than one flower.
4. Don’t dwell on the exceptions! Most plant species are quite variable. Throughout the keys you will see qualifiers such as “often,” “usually,” or “mostly” to account for the lack of uniformity.
5. After reaching a tentative identification, read the descriptions and look at the illustrations to confirm that you are correct.
6. Make your observations slowly and carefully, especially if you are new to “keying out” plants.
For convenience the initial key below divides aquatic plants into six groups based on growth habit: emergent, floating-leaf, submergent, etc. The next step is to use the section keys to arrive at a genus and finally the keys to each genus, which are found in the succeeding chapters, to name a plant to species.
The keys assume the plant’s appearance under normal water levels. Many species assume unusual growth habits and/or morphological forms when the water is unusually deep or shallow. Some species will appear in more than one habit category, since they assume more than one form under normal conditions. Whether a plant is considered emergent, submergent, or floating depends on whether the main body (leaves and/or primary axis) occurs above, below, or at the surface of the water. Species that produce only their flowers above the water surface are not considered emergent or floating-leaved. Be aware that exceptional variations may, and occasionally do, occur.
Key to Growth Habit Groups
A. plants without distinguishable leaves and stems, or tiny floating plants with minute leafy stems (leaves less than 1 mm long) ........................................................................... Thalloid and/or tiny floating plants, p. 163
A. plants with distinct leaves and/or stems; leaves, if present, more than 1 mm long
B. plants with at least some leaves and/or stems held above the water surface ........................................................................... Emergent plants, p. 29
B. plants entirely under water or with at least some leaves floating at the surface
C. plants with at least some leaves floating on the water surface ............................................................. Floating-leaf plants, p. 143; or some pondweeds (Potamogeton spp.), p. 186
C. plants with leaves entirely or mostly under water, or buried in a sandy or peaty substrate; at most flowering above the surface or with only leaf tips floating at the surface; or stems sometimes leafless
D. underwater leaves broad to very narrow, but not finely divided
E. underwater leaves attached to an elongate stem, or submergent stems sometimes leafless, plants rooted or free-floating ............................................................. Submergent leafy-stemmed plants, p. 183
E. underwater leaves in a basal rosette, plant rooted on the bottom ................................................ Submergent rosette-forming plants, p. 241
D. underwater leaves very finely divided ............................................................................. Submergent plants with finely divided leaves, p. 255
Figure 2.1. Stems jointed, tiny scalelike leaves in whorls at the node (×3/4).
Key to Emergent Plants
A. emergent leaves long and narrow, their blades more than 7 times longer