This guide will tell you where to find more Mallow sweets in Hogwarts Legacy.
You will need to use Mallow sweets to activate the Merlin trials. So go to Hogsmeade and head to the Dogweed and Death Cap shop to buy the fertilizer.
Afterward, go to the Magic Neep shop to buy the Mallow sweet seeds. You won’t be able to do anything with seed early on in the story; you need to progress far enough until you get to the upper class.
Then just start to grow stuff in there, and it is recommended to go a bit further until you have the room of requirement. The room will allow you to grow stuff a lot easier. After that, fast travel to it from the secret room section, then go to your room of requirement and raise your pets. You can also make your potions and plant your stuff. Just empty the pot if you’ve got something in there already you want to plant your mallow sweet seeds and then fertilize it.
It will put the yield up to six, you can just keep it in there. Keep using the seeds, go out and do some Trials, then come back every 10 minutes to get Mallow sweets.