Can I sell the same flower by the stem and by the bunch?

Yes: give the flower one row per way you sell it, and each planting counts its revenue in the unit you choose, stems for the florist, bunches for the market.

In Crop yield, click Add new chart item for the same flower with another Harvest unit, such as a bunch of 5 stems, then its yield per bed and its price.

A new planting starts from the first row of its flower or cultivar. To sell a planting the other way, pick that unit in the Harvest unit list of its Harvest And Revenues tab.

The steps are in Set yields and prices for your flowers.