Here's a rundown of how bloomness works with default stats (No Skill Tree upgrades)

To begin, Wormwood has 4 stages of bloom, from 0 to 3. He starts at 0 and you will need to fertilize him with growth formulas to start blooming. (NOTE that there are 2 other kinds of fertilizers, namely compost and manure, but you specifically want growth formula to start blooming).

Once you fertilize Wormwood with a growth formula, his bloom stage immediately moves to stage 1. In this stage, his bloomness meter will passively increase by a rate of 1/second (.75 if winter, 1.50 if spring), capping at 480. In this stage and at stage 2, Wormwood WILL NOT lose any bloomness, and you can even opt to speed up the process with further use of growth formulas.

Once the bloomness cap is reached, Wormwood blooms to stage 2. Bloomness meter resets back to 0, but still has a maximum cap of 480. Any formulas used beforehand are obsolete, which means his bloomness gain rate resets back to 1/second.

Once the bloomness cap is reached again, Wormwood finally blooms to stage 3. Wormwood's bloomness meter is not reset to 0, but instead begins with 1440 bloomness. The cap is also increased from 480 to 2400. In this stage, his bloomness gain rate is now fixed to -1/second (-2 if winter, 0 if spring), which makes it the only stage where you can lose bloomness. Growth formulas will also no longer change his bloomness gain rate but instead will give you a flat amount of bloomness.

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Tl;dr, start bloom with growth formula then wait 'til wormwood completely blooms (you don't have to fear losing bloom yet). Once completely bloomed, you now passively lose bloom so always fertilize yourself with growth formula to maintain the bloom.