When is the best time to control unwanted plant growth ("weeds") is before they produce seeds and then it depends on whether the unwanted plant is an annual, biannual, or perennial. Control of some is best in the spring while others are best controlled during the summer or fall. The most common broad leaf "weed" killer is something containing 2,4,D and Dicambra, both of which are known to have serious environmental problems.
Unless your neighbors control the unwanted plants on their property you will see new growth in the future even if you totally eliminate them on yours.

Here is a link that might be useful: About 2,4,D and Dicambra

Wouldn't a good pre-emergent help with the neighbors uninvited guest seeds? Or the birds undigested goodies for that matter as well? Your message makes it sound like any effort is futile so I wanted to see if I'm understanding your concern.

As to your question, kimmsr does makes a valid point of understanding the best time, and taking action then. To large a weed is tougher to kill than a small, newly emerged one.

Here is a link that might be useful: Control options for common Minnesota lawn and landscape weeds

Greetings,

Thanks for the responses. My point in the original post was to convey I won't be able to permanently knock out the weeds since others in the vicinity do not treat and kill off the weeds and seedlings. I can do so on my property, however. Does anyone have advice on what specific product might help with nettles, thistles and mustard seed?

I would not want to use herbicides in woods. I have found that regular cutting e.g. with a brush cutter, will control these weeds. By cutting them repeatedly when under a foot high they will never get a chance to seed and other things will have a chance to grow amongst them. I have all 3 in my wood and I just have them cut two or three times a year, starting as soon as they reach 8 - 12 inches. Herbicides will kill all broad leaved plants, including any desirables. You can't specify one which only kills those three.

Nettles and thistles are perennial and will regrow, although weaker. Garlic Mustard is annual so repeated cutting before it can seed will eventually get rid of it.

All of the plants we refer to as "weeds" are wildflowers in another setting and since they produce seeds so prolifically "getting rid" of them is nigh onto impossible. There are people that do spend a ton of money on products that will kill off unwanted plant growth and you see that in many of the lawns that also need large inputs of money for to sustain the lush green growth and to suppress plant diseases.
On that much ground the nest means of "weed" control is mowing.

Use a weed whacker in early spring and then every couple of weeks thereafter.