Hmm... I don't know about herbicides. I personally try to not use them much. You said the area is only 1/4 acre, that doesn't sound very big at all, I mean our house is on half an acre and if I had a pumpkin patch that big I'd get busy pulling the weeds using a hoe and by hand. Pulling the weeds will make a noticeable difference in how fast your pumpkins will grow no question about it. I don' tknow what kind of pumpkins you planted, the time it takes for them to ripen varies, but it is usually somewhere between 90 and 110 days. It sounds like right now you're somewhere around maybe 60 days or so, and I'm pretty sure september will be a decent month up there so I think it's possible for them to ripen. Also, you can leave them over there until late October, maybe even early November, so I think you'll have pumpkins. Just make sure they get water and they're properly fertilized. Pumpkins like a LOT of fertile soil. I make my own fish fertilizer. It is excellent stuff to use on cucurbits. Ultimately I think someone in your area might have a better answer since they're more familiar with the weather patterns there. I'm on Whidbey Island, WA

You can use Round Up to get rid of weeds, but pulling them or cultivating with the tractor is much better in my opinion. If you do use Round Up be careful not to touch a drop to your plants, pumpkins are very susceptable to it, and it isn't good to get on anything that will be eaten. I don't know when your frost date is, but pumpkins can be left in the field for a while after frost, although they may not ripen any further. If you haven't fertilized them yet, it should be done particularly if they are flowering. I would use something like 12-24-12, maybe a couple of tablespoons per hill, although 1 now and 1 a week later if they still look weak is better. Organic compost would be better, but it is kind of late if they are already flowering.
Good Luck,
Tom