With few exceptions, the most aggressively invasive weeds here are exotic. In other words, they are not native. They came from other regions where they were likely compliant participants of their respective ecosystems. At home, where they must compete with other members of their ecosystem, they may not be so aggressively invasive. Ecology is the opposite of a home field advantage.
Exotic species become invasive weeds in foreign ecosystems for a variety of reasons. For some, the climate is more favorable. Some grow and proliferate more freely without diseases, insects and animals that troubled them back home. There are also several that simply compete more aggressively for space and resources than native species are accustomed to. It is a jungle out there.
Most invasive exotic species are annuals. Many are biennials or perennials. Some are vines, shrubs or even trees. Most were imported intentionally, for a variety of reasons, and then naturalized. Forage and cover crops were some of the earliest of exotic species to become invasive. Other invasive species escaped from home gardens. Blue gum eucalyptus was imported for wood pulp.
Regardless of their origins or physiological forms, invasive species are weeds. They compete for the same resources that desirable plants use. They impede on the aesthetic appeal of gardens and landscapes. Some types of weeds become hazardously combustible. Even if not directly problematic, invasive weeds disperse seed that can be problematic nearby. Many disperse stolons.
Most weeds start early and grow fast to get ahead of their competition. They are more active at this time of year than at any other time. They are also vulnerable. While the soil remains damp from winter rain, they are relatively easy to pull intact. They have not yet dispersed seed for their subsequent generation. Later, they are likely to leave behind seed and bits of roots that can regenerate.
It is important to pull or grub out seedlings of unwanted shrubbery and trees, as annual weeds. They are likely to regenerate if merely cut.
Tony Tomeo 
We find this too in my “frozen north” climate. The invasive plants are the first to appear or show color. Luckily after our snowy winter the ground has plenty of moisture to let us get most of these out with ease–with the exception of bittersweet, a vine, whose root extend to middle earth, I think.
Karla
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I think that some of those weeds that are difficult to pull extend their roots to the opposite side of the Earth, where someone else is trying to pull them up at the same time. That is why they are so difficult to pull.
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I’m very reluctant to use herbicides but less so early in the season when it is easier to target weeds that green up early.
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I SO want to use herbicides of some sort, but the situations where they would be helpful are the same situations in which herbicides are not allowed! Some of the landscapes are within riparian zones. Even if they weren’t I just do not trust herbicides here. We have used vinegar, and even fire.
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I’ve heard both can be effective for some things.
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Yes, but no so much for the perennial stoloniferous grasses that we use them on. Fire burns the foliage, but the process must be repeated very regularly to deprive the stolons of foliage long enough to die. Vinegar, however, is effective on the mosses, which may not seem like much, but is useful for some of the stone walls here that look like Chia Pets.
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Oh yes! And you had to show a photo of a thistle. 🙂
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