Seeds: Everything you need to know:
You need to grow rare seeds to get any rare plant. It is random which plant you will get out of a rare seed. But it might not hurt to raise your vitality because the game says the chances are better to get the rarer seeds when your vitality is high (for an explanation of vitality see Vitality File ). No need to hurry though. If this is a task, it will wait (also across multiple levels) until you're done.
The seed packets have printed on them, "Guaranteed 15% rare seeds" if they are Mystery Seeds, or "Guaranteed 60% rare seeds" if they are Enigmatic seeds. Extraordinary and Spectacular seed packs have printed on them, "Guaranteed 100% rare seeds".
Mystery, Enigmatic, and Extraordinary seed packs contain more cheap/low-vitality rare seeds and fewer expensive/high-vitality seeds. Spectacular seed packs contain more expensive/high-vitality rare... seeds and fewer cheap/low-vitality rare seeds.
Also, the Extraordinary and Spectacular seed packs include a Miracle-Grow type of fertilizer to get your seeds to grow faster. Enigmatic seed packs do too, but with less fertilizer, so they grow faster than Mystery seeds, which have no fertilizer, but slower than either Extraordinary or Spectacular seeds.
The different seed types have different chances of giving you a rare plant versus an ornamental plant:
Mystery Seeds: Are affordable but get more expensive on higher levels. They have a 15% chance each of resulting in a rare plant.
Enigmatic Seeds: Are a bit more expensive (10000SD + 6000Alg) but grow faster. They have a 60% chance each of resulting in a rare plant. Odds of getting a higher value rare (like the mermaid's fan plant) are slightly better.
Extraordinary Seeds: They are expensive (10 Pearls) and are ready grown almost immediately. Every extraordinary seed results in a rare plant. Odds of getting a higher value rare are just a bit better than for enigmatic seeds.
Spectacular Seeds: These are very expensive (45 Pearls). But they are very different. All three other seeds have a very low chance of getting high value rares. The Spectacular Seed has actually a higher chance of getting high value rares and only a low chance of getting low value rares. Because of this Spectacular Seeds are much more worth the pearls compared to Extraordinary Seeds when you head for high value rares.
When to use which Seeds:
Mystery Seeds: From the start up to (roughly) Level 40. At some time they get expensive and you may have enough left for Enigmatic Seeds.
Enigmatic Seeds: Need less place & time than Mystery Seeds. Less place because of the higher success rate. You can keep using these perhaps until you switch to Spectacular.
Extraordinary Seeds: They're expensive and because they're fast, more for the lazy ones. Use them if you've got plenty of pearls and/or need any rare grown fast (i.e. for a task)
Spectacular Seeds: These get especially useful in the higher levels. These seeds are the best way to get high value rares. Usually you begin to use them more often when you've got bigger amounts of algae to feed your rare fish and therefore have some more pearls to spend. That may be around level 48 when you've grown many red christmas tree plants.
For the thrill, you can use these seeds also earlier (perhaps from level 30-35 on) but don't get too frustrated when they "fail". Its all statistics!