sfrose has explained it very well in her Seed Catalog in the Plant Tycoon Guide Section. To make it easier for you to see it, I've copied it here for you:
Tier 1 foliage and flowers can be found in the starter plants, seeds, and common seeds in the Supply Store. They will survive in all soil levels.
Tier 2 foliage and flowers can initially be bred from Tier 1 foliage and flowers. They can also be found in the endangered seeds in the Supply Store. They will survive best in upgraded soil, but with TLC can survive for some breeding programs at the basic soil level.
Tier 3 foliage and flowers can initially be bred from cross-pollinating Tier 1 and Tier 2 foliage and flowers. They can also be found in the endangered and extinct seeds in the Supply Store. They will survive only in the best soil, but with TLC some can survive for some breeding programs at the first upgraded soil level.
Cross-pollinating Tier 3 plants with other Tier 3 plants, or combinations of Tier 1, 2 and 3 foliage and flowers may result in new and exciting species for you to discover.
Tier 4 foliage and flowers are extremely rare! They can not be initially bred from cross-pollinating, but special chemicals are needed to find them. However, they can also be found in the extinct seeds in the Supply Store. They will survive only in the best soil, and are not known to be able to survive in the lower grade soils.
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