A century and a half ago the Victorians attached specific meanings to individual plant species.  A red rose resembled love, an oak 'liberty', a lotus flower 'eloquence'.  
More recently we have attached new meaning to flower giving based not on emotion but on the scientific evidence that flowers generate emotions in us.  

Here are some of the effects of flowers, based on modern science:

  1. Flowers have an immediate impact on happiness. All study participants expressed "true" or "excited" smiles upon receiving flowers, demonstrating extraordinary delight and gratitude. This reaction was universal, occurring in all age groups.
  2. Flowers have a long-term positive effect on moods. Specifically, study participants reported feeling less depressed, anxious and agitated after receiving flowers, and demonstrated a higher sense of enjoyment and life satisfaction.
  3. Flowers make intimate connections. The presence of flowers led to increased contact with family and friends.

With Valentine's Day just around the corner we are reminded that flowers are an appropriate method of relaying a message, whether it is an expression of love or something else.

Here are my recommendations, based purely on personal experience (not science):

  • For when I fed the kids dog food (which I actually did…) a special arrangement featuring dogwood cuttings and doggie biscuits.
  • For when I forgot Valentines: a big arrangement with an invite to somewhere special….create an event with the flowers.  
  • Her [or his] favourite flowers… favourite fragrance…. Favourite place to eat….a promise to help her in the garden pulling weeds.  Watering when she is tired. Mulching to reduce the work (watering and weeding).
  • Flowers for mother in law: seeds of honesty plant….forget me nots…..fragrant hyacinths.
  • Young daughters from Dad: small pot of bulbs in bloom for their bedroom
  • Dad to son: flowering shrub with pruners
  • Single girl friend to single girlfriend: wine with... nice flowers.  
  • For when I screwed up: sunflowers.  Daisies. A happy, bright, sun loving flowers.

You can, of course, attach your own interpretation to the flowers that you give.  How accurately you are able to match the occasion to the flowers is not nearly as important as the fact that you thought of giving them in the first place.