If you put weaners onto very overgrown ground they can get an impacted gut from too much greenery. Pigs must have a dry lie, shade or a wallow, good fencing ( do you have a proper outer fence to contain escapees?) and a 24-hour supply of clean water. You also need to think about how you're going to load them for the market/abattoir when they're ready for slaughter. Around here piglets weaned in April to June are ready for slaughter in early Autumn, so you don't have to worry about the water supply freezing in the winter and the pigs hauling themselves around in icy mud. The saying with thistles is "Top 'em in July, then they'll die." If you can stop them seeding for a couple of years it will help.