Floriade is a must for travellers around mid September to October. Free to visit and wonder for as long as you like during daylight hours. Flowers may not be your thing but there are amusement rides, demonstrations, crafty stalls including wooden iphone holders and fudge. You may even see a dinosaur or two.
Nightfest is another aspect with viewing the gardens at night with added food stalls. I dare say the pollen count would be lower too. This is a paid event and runs rain, hail or shine.