Alchemilla mollis

No one's going to say anything? Really, no one's going to say it? I mean, I'll say it if no one else will because someone's got to...
All right, fine.
Alchemilla mollis (Lady's Mantle) is a menace of a weed no matter how you look at it. I cannot be rid of it. It devours gardens while my back is turned. I've scratched futile days away clawing at its rhizomes only to find, upon my next visit, it has returned in viral numbers. It seeds itself with a profusion and desperation usually reserved for certain notorious annual weeds. Yet everyone turns a blind eye to these truths, defending A. mollis as though it were a troubled youth who means well, no matter how many schools he burns down. Yes, the foliage is pretty, and it prettily holds dew and raindrops in its leaves, but once you've held a thunderstorms' worth of rain in your Alchemilla leaves alone, the novelty wears off.
Everyone loves this plant at first sight... and second and maybe third sight. Then it flowers and people aren't quite sure how to feel about the sloppy yellow inflorescences which seem to belong to another, quite different plant (who would have guessed this thing belongs to the Rose family). Then it explodes, producing a million seeds which germinate at about 99.9% success rate no matter what the soil or exposure, and people nervously reason that if they liked a plant enough to buy it in the first place, then having a million more for free must surely be a good thing. Then they sit in passive denial for a year or so until one morning they open the front door and can't find a way through the garden because there is no garden anymore... only an uncrossable sea of Alchemilla mollis... and they call Garden Search and Rescue, aka yours truly, who is airlifted in and charged with the unenviable but heroic task of finding safe passage through the sea to rescue them before food and water supplies run out.
Do not fall in love with this plant, no matter how delightfully blue-green the leaves seem at first glance; do not give away your garden and affections for the price of a pretty dew-drop. This Lady's Mantle is ravenous.
There, I said it.

Nightmare Weed

All right, I'm curious. I am engaged in an ongoing debate/dialogue/cursing fest with myself and coworkers regarding which of many is the worst weed/invasive species to remove. I tend to go back and forth between Oxalis (oh those pretty little shamrocks from hell) and bindweed (Convolvulus arvensis, whose explosive germination any time earth is moved from one spot to another seems to support the centuries-old theory of spontaneous generation).
Less common, but more likely to spawn genuine nightmares is Alstroemeria. I kid not, after a whole afternoon of digging and searching for these elusive, grave-depth fleshy tubers I had a restless sleep in whose dreams I was likewise digging endlessly, but with my spade repeatedly striking and uncovering unexpected objects like full bottles of insecticide spray. This was disturbing in a way that I can't quite justify or explain. Last week I returned to a garden from which I meticulously removed every last trace of Alstroemeria to install some plants. Lo and behold, when digging holes at a depth of 12-18", I began to unearth those wormy white clumps that would be next years crop. For all I know, these tubers penetrate the Earth's mantle and can be found well into the core of the planet; I am afraid to ask whether oil-drilling outfits have contingency plans for encountering abnormally large Alstroemeria clumps at depths over 2 miles. I can no longer simply smile and compliment someone's lovely bouquet of store-bought Alstroemeria without choking back a grimace and repressing a handfull of insidious memories.
Thus, after some delay, to my aforementioned curiousity. I would like to know what other people's nightmare weeds are, and for what reason. Please feel free to leave your opinions in the comments section. If I get enough different responses, I would like to put up a poll so others can vote on their least-favorite weeds.

Out With The Ferns, In With The...Oxalis?

My working definition of a weed is any volunteer plant that I am routinely asked to remove from a given garden. Since I have no garden of my own, the only weeds I can define for myself are the ones tenacious enough or of seed omniscient enough to sprout up in my numerous container plantings. I am not fond of these weeds, as I'd like to think one of the few benefits of not having a garden would be an absence of menial maintenance tasks such as weeding. Therefore, I have no great attachment to the grasses, dandelions and liverworts I disgustedly pluck from my pots when they begin to obscure my beloved plants.
It is considerably more difficult to align my weed definition with my gardening conscience when I am in a certain client's garden. Especially when said client is instructing me to "dig out those pesky ferns" because they are "everywhere". The ferns in question are two or three lovely western sword ferns (Polystichum munitum) of modest size and excellent health which, despite my being able to count them on one hand, are apparently everywhere. Now let's never mind that these are native to this area and so were springing up as "weeds" eons before humans slapped up their poorly-proportioned million-dollar ramblers-with-a-view on the site. Let's instead consider that this client puts her foot down on a few sword ferns, branding them weeds, while all about her sprout the immortal heads of at least a dozen terribly invasive species which truly are everywhere and which she herself introduced into her garden. Oxalis, Alstroemeria, Aegopodium, Ajuga, Digitalis, Aquilegia (not the native), and Angelica abound, to name a few.
I propose that we show some tolerance for sword ferns (which, by the way, are quite easy to remove and relocate if necessary) and show some discretion in the aggressive plants we willingly bring into our gardens. Future home-owners and gardeners alike will thank us.