Decide what you want from your garden and plant accordingly. Both King Parsley and Evil Magma Peas are popular plants due to their drops. King Parsley produces Amber and reseeds itself. Evil Magma Peas produce mega snacks and reseed themselves. Pick up one or two of each (either as a drop or in the Crowns Shop). W101 will occasionally have a double harvest event when these plants produce an extra seed. Over time, you can make a very impressive garden full of them.
Amber is very hard to get, and is used to craft spells (the ones the Loremaster very rarely drops).
Mega snacks allow you to rapidly train up new pets - a handful of these and some brief training will get a pet to Adult.