Yes. Use a watering can with little holes that spread the water and apply directly to the base of the hosta. Best time to do that is just as they are emerging, but later is better than not at all. If it's regular household cleaning ammonia, use 1 part to 10 parts water. The only thing I've not bee able to find out w/o looking at a bottle (not handy at the moment) is the strength of our household ammonia and whether it's the same as what you will find in France. When I get a chance, I'll look for a bottle and let you know what it says.
