Plant taxonomy aside, there are really two main plant types when we are considering management: annuals and perennials. At this point all those whose eyes glaze over when the Latin genus names start rolling out should be paying attention, and gardeners and home landscapers probably already get it. Much of the decision about what to do rests on whether a plant reappears annually from seeds or other resting stages (propagules) or comes back from root stalks or stems (or never really dies back in winter). Perennial plants are easier to deal with (just like in your garden or lawn); if you kill the plant, it won’t return from its vegetative parts. For annuals, however, killing the plant just buys time; seeds, winter buds, turions, and other structures germinate and form new plants, usually in the following spring. Management strategies therefore have to consider whether the target plants are annuals or perennials, when they produce propagules, and when growth begins.
If one is applying drawdown, it can be effective on perennial plants like the milfoils, but will have little impact on annuals like naiad or most pondweeds. In fact, drawdown often stimulates propagule germination, so a shift from perennials to annuals can be expected over a period of years with continued annual drawdown.
If one is using herbicides, killing the plant before it can drop seeds is essential to eventually gaining control over an annual, but the seed bank may be large enough to allow recovery for years to come. If a contact herbicide is used, the root system of a perennial is usually unharmed and will allow plant regeneration. Systemic herbicides, which move throughout the plant and are intended to kill it all, can provide more lasting results with perennials, but are more expensive than contact herbicides and may be a waste of money for annual plant control.
Benthic barriers kill nearly all plants over which they are placed, but if the barrier is removed, propagules from annual plants are likely to sprout. Hand harvesting practitioners know that it is essential to get the whole plant out, roots included, if a perennial species is to be controlled by that means. Dredging may be the only all-purpose tool for rooted plants, removing the plant and the seed bank, but at a very high cost with a lot of permitting, so we don’t see this approach used much.
Some plants apply a mix of perennial and annual strategies, making them harder to manage. Bigleaf pondweed produces seeds but does not die back completely in most winters and survives many drawdowns and herbicide treatments. This is a native species in New England and is not a major nuisance plant in most cases, but it has created problems for swimming in some lakes. Many perennials do produce seeds, but viability tends to be low; they don’t depend on these for annual persistence, but it does mean that removing a stand of a perennial plant may not be a one-time job.
So be sure to know which plants apply which strategies when considering control; success may depend upon it.
Curlyleaf pondweed, an annual
Eurasian water milfoil, a perennial