So when do we know when to fertilize and what kind of fertilizer to use? This is a Rosemary tray and as you can see, some of the plants, they look nice a perky and when I give them a little tug they're not really letting out. If they are loose then they are probably not rooted but if they're given a nice little tug, they're probably rooted. These are still wilted so you can tell these are probably aren't rooted at all. They're not really doing anything. And then if I carefully tug out one of these, let's see if I can get one with the roots to come out, without breaking them. There's one, you start to see some roots. I have to carefully pull these out because you could break off the roots. But when I start seeing enough of the plants starting to throw out a few roots I might start with a half strength fertilizer. You can get fertilizer at the garden center. Just make it half of the full strength that they tell you to make. There's all different types. They probably have ones for propagating but a general type of fertilizer is 20-10-20.