You can make things a little faster by right-clicking a plant you want to tend and then pressing 0 2 0 on the numpad (0 is default key for interactions; 2 moves the cursor down)
If you put fertilizer in the top left slot of any bag you can fertilize in the same way by hitting 0 three times in a row on the numpad.
Most plants will wilt (turn purple) 48 hours after not being tended. They will wither (die) 24 hours after turning purple without being watered.
A few plants will wilt after only 24 hours (Blood Pepper, Gysahl Greens, Jute, Krakka Root, Royal Fern, Thavnairian Onion, Minions) and will wither roughly 24 hours after wilting without tending. Exact times have not been tested thoroughly.
Plants will not wither no matter how many times (or how long) they wilt during the growing process. Plants also continue growing while they are wilted. Testing results can be found on the Tending Analysis page.
You cannot crossbreed with withered plants.
As of Patch 4.0 plants do not wither after they are ready to be harvested. (Source: 4.0 Patch Notes)
Good Samaritans with Watering Cans
It is possible to tend (water) the plants in any outdoor garden regardless of FC membership, private house ownership, or private house tenancy. (It is not possible to harm plants through tending them too often, so there is no restriction). This means: if you see a neighbor's garden has withered (gone purple), you can water it for them!
This also means that Discord members can help water your garden in case of emergency, as long as someone has (or can make) a character on your datacenter/server.
Replanting Next to Harvestable Crops
For short grow time seeds (such as Earthlight or Voidrake) you can harvest, replant, harvest again several times and get seeds each time they are harvested before the long grow time seed has finished. You don't have to wait for the other plant to finish first.
In fact, you can even leave things planted forever (finished, sparkly) and just keep using other plants to cross with them for seeds. This is actually a good strategy for really difficult plants that you can't replace easily (like Blood Pepper).
Seed & Soil Gathering
When you're gathering seeds or soil that are hidden on a node, you don't have to clear out the nodes as you check them! You can simply open the node, check for what you want, and, if it isn't there, go onto the next node! If you look up on your minimap, you can see the nodes behind you in your circle 'resetting' or changing locations, meaning that they've reset!
Leaves of Absence
Garden patches and flowerpots with growing plants may be placed into storage. This freezes the timers on the plants, meaning they will neither grow to completion nor wither while in storage. (Source: Patch 4.1 Notes)
Obtaining Grade 3 Topsoil with Poetics
Correct Use of Thavnairian Onion