It only takes one neighbor who doesn't take care of his yard to undo your hours of labor to keep your lawn weed-free. You may be tempted to sneak into his yard and take care of the weeds yourself, but you have some options from your own side of the property line, including barriers and improving the health of your own lawn.
Talk It Out
Instead of assuming your neighbor is thoughtless or doesn't care about your lawn, bring up the topic of her weeds. Let her know you noticed she has a few weeds taking over in her lawn. Offer up your advice for stopping the weeds before they spread. She might not realize she has a problem or she might not know how to get rid of the weeds herself. If she seems unresponsive to your suggestions, let her know that you don't want her weeds spreading to your property. You'll also want to chat with your neighbor if you decide to make any landscaping changes along the property line. Even if the project takes place on your side of the line, your neighbor has to also see the results. Check with her to ensure she doesn't mind what you're planning.
Put Up Fences
Physical barriers between the two properties can slow the spread of weeds. A fence is a suitable option, but you'll want a fence with few if any gaps. A chain-link fence won't do much to stop the spread of weeds. Instead, opt for a wooden privacy fence with the boards butted up next to one another. The fence can keep weeds from growing onto your property. It could also block some of the weed seeds that travel through the air.
Landscape Property Lines
If you don't want to put up a fence, you can slow the spread of weeds into your yard with the types of landscaping you use along property lines. Weed aren't able to grow easily through rock and mulch. Instead of letting your grass grow right up to the property line, dig up the lawn and landscape with a rock garden, or install raised beds that create a physical barrier for the weeds. If you already have planting beds along your property lines, install a landscaping border around the bed to keep weeds out. Mulch the beds so weed seeds aren't able to take root between your plants.
Use Pre-emergent Herbicides
You often won't know if your neighbor's pesky weed seeds made their way into your lawn until it is too late. Using a pre-emergent herbicide in early spring around the time the seeds germinate can stop those hidden seeds from growing into weeds. Corn gluten meal is a natural product that works as a pre-emergent herbicide by interrupting the normal root development of the weeds. Apply the corn gluten meal at a rate of 12 to 20 pounds per 1,000 square feet using a spreader.
Keep It Healthy
The health of your own lawn is a factor in how easily your neighbor's weeds can take over. If you have a dense, healthy lawn, weed seeds can't take root and sprout as easily. Watering to supplement rainfall so your lawn gets at least 1 inch of water weekly helps the growth of your lawn. Mow only as much off the grass as necessary. Leaving the grass a little longer blocks out the sunlight weeds need to grow, so mow accordingly. Even if your neighbor's weed seeds make it into your lawn, your lush lawn can often hold off growth of the weeds.