I've done some magical planting this year. Many ingredients that compost well in soil also have plenty of magical uses; it's a perfect setup to grow your magic and feed your plant.
Growing plants in magical substrate is a way to transfigure energy. Whatever energy you use to feed the plant is then utilized for growth and turned into something beautiful, medicinal, or edible. It transforms death into life, and is an excellent way to watch the Wheel in action. It is also a form of passive spell casting - you don't put in much more energy than the mundane work needed (unless you want to, of course) and the plant does all the work. It's a good way to build a plant ally as well.
Some things I use in my plantings (by no means an exhaustive list):
Eggshells - opportunity, growth, the unknown, birth
Coffee - stimulating, re-energizing, a good connection to what is a daily ritual for many, you can also use another's old coffee grounds as a taglock (what they have consumed the plant now consumes)
Bone dust - need I say more? Connection to the animal that passed, death in general, necromancy, changing the tides of a static path/dead end
Ashes - any ashes! Ashes from old workings, ashes from burnt objects, burying curses and creating opportunity for yourself, the list goes on eternally
Worm and insect casings - very nutritious, good for feeding spells, good to network or protect your land
If you have others, I'd love to hear them!