How much work does it take to get rid of a weed? Will I need to treat this population again next year? These are simple questions when managing weeds and the answer falls into a general pattern.
In general, a single treatment on annual weeds with short-lived seeds can be effective, and a treatment next year might not be necessary. In annual weeds with a longer-lived seed bank the treatments can be effective on current growth but not visibly effective the next year. For perennials single treatments can be very effective and they can be successful for many years. Proper identification is paramount and a knowledge of the biology of the weed also provides tremendous help with eradication measures, especially if you are pressed for time spent managing weeds.
Annual weeds
The goal with managing annual weeds is stop seed production. Annual plants spend part of their life as seeds in the soil, the other part of the year is spent as the herbaceous plants we readily recognize. If annual weeds do not produce seeds that generation of plants is lost. Some seed is stored in the soil as a seed bank. Among many California weeds with a short-lived seed bank are grasses in the genus Bromus and rattail fescue. Brome seeds tend to remain viable in the seed bank for 1-3 years, depending on the species. In addition, the vast majority (>90%) of brome seeds germinate every year. Thus if the single treatment produced high mortality, the population of bromes will be at least 90% less next year, all other things equal. This is one of the reasons brome populations tend to crash after a very dry year. Nature provides a single year treatment (drought) and since it reduces 90% of the seed bank the population crashes. This is enough of a reduction to let wildflowers bloom the next year, assuming we receive enough rain and wildflower seeds are in the seed bank
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Some annuals have a long-lived seed bank. Filaree and mustard seeds can survive for 3 years and yellow starthistle seeds can survive for 6-10 years in the soil. A highly effective treatment in a single year will result in poor long-term control, there are simply too many seeds in the seed bank that germinate the next year. Managing annuals with long-lived seeds for only one year will result in a decrease in the seed bank but it will not be enough to qualify for success. With these weeds multiple treatments over multiple years is required.
Perennials
In contrast to annuals some perennials have very short-lived seeds other seeds can survive a long time and woody perennials also grow much slower than annuals. A treatment in a single year will result in long-term control of some perennials, partly because it can take a decade for a tree to regain its former canopy and produce seed.
However, some perennials resprout vigorously, such as tree of heaven or perennial pepperweed, and will regain their former canopy in a few years. Only complete mortality will prevent resprouts in later years.
A single treatment can be extremely effective on many woody perennials including palm trees, tamarisk, russian olive, and eucalyptus. This comes with two caveats the tree must be completely killed and any seedlings will eventually need to be treated.