A lot of people will tell you to spray the weeds off before planting a hawthorn hedge.
Don't bother. Use woven polypropylene weed prevention fabric instead. Cut the undergrowth short, put the fabric down where you want to plant the hedge and weigh it down with stones. If you want to be really tidy, push a strip about 2" (5cms) wide of the fabric into the soil with a spade down each edge and at both ends (there is a good film on how to plant a native hedge on our site).
You slit the fabric where you want your hawthorn plants and plant as normal. No weedkillers, no regrowth. And no need to water as it acts as a mulch as well. Over the 2-3 years it takes your hedge to establish this is an amazing labour saver and will cost you less than the weedkiller bill over the same time.
And, if you need them, we can supply the hawthorn hedge plants too....
Sit back, relax and watch your hedge grow. Enjoy.