Hi.

I've been growing plants for years, and the most important thing to remember is to do *everything* through the Plant Care Menu that is accessed by double-clicking the plant bowl after you've filled it with dirt. Yes, this can and *should* be done *before* you add any water to the "bowl of hard dirt". The reason for caution is there's really no recovery from "squishy"... yuck!

No way to save it then, nothing to do but empty the bowl and start over with fresh dirt.
Never at any time add more than 2 of anything, not water or any of the 4 potions.
Remember that the potions are all "Greater":
-o- Greater Poison (GP) - You'll use a ton of this if you're at all serious with your plant growing;
-o- Greater Cure (GC) - The ratio of use for this is approx 1 keg for every 4 kegs of GP;
-o- Greater Heal (GH) - If you do it right, you'll hardly ever have to worry about using this;
-o- Greater Strength (GS) - Some people swear by GS, use it every day, others (like me) have dropped off the GS bandwagon and just wing it, only using GS when plants are weakened.
As to potions, go straight for the kegs. My gardener has a pouch with a keg of each plant potion in it, and she places it in her backpack. No need for a bottle, when clicked, the Plant Care Menu right-hand buttons will take 1 dose at a time straight from the keg.
Celticmoon, the most wonderful site on UO plant growing ever put together no longer exists

, so guess you'll have to settle for the links given you by the others; as wanting as they are, they're better than nothing.
When cross-breeding, remember "Plain" always wins. Any plant of any color when crossed with a "Plain" plant of any type will yield a "Plain" seed... though it might be a totally different plant from either of its parents, it will always be "Plain".
Hope you enjoy growing plants as much as I have over the years.
Be well - Pax