I disagree with the remark that people can only convey feelings with flowers if the recipient knows what the flowers signify. Perhaps this is true with the insensate or automaton, but if a person has any emotions and sentiments at all, he/she will have an intuitive and natural reaction to the beauties of Nature. Consider, for instance, the design and color of different flowers. Do we really need to know what the yellow or pink rose symbolize to enjoy its beauty and fragance? No. But, of course, there is added enjoyment and meaning that the yellow rose, for instance, symbolizes friendship if one receives these as a gift.
The best flowers are those that match the love interest's personality. It may be the proverbial red rose or it may be a less well-known flower. A red rose is a great flower to give if you wish to stick with tradition and what it represents. A red rose is also suitable to give to someone who has a fiery red passionate personality.
However, other flowers can speak to one's personality as well. For example, the amaryllis represents "splendid beauty" so if you're enthralled by your love one's beauty give them this type of flower. Daisies and Chrysanthemums represent fidelity, so they are appropriate to give to a loved one who exhibits this quality. The Lisianthus flower represents an outgoing personality so it is suited to give to one who has an exuberant and extroverted personality.
Different roses symbolize different things. Red ones are passion, pink ones are love, and yellow ones are friendship. Of course, most people have a favorite flower. I love orchids. Orchids make me feel happy. So if you want to make someone feel good with a flower, you have to know the person to choose the right flower!
We can only convey specific feelings with specific flowers if both we and the recipient know what a certain flower means. There is no inherent meaning in a rose or a chrysanthemum, for example. Many people say that roses symbolize love, but this is only true if the receiver of the flowers understands that symbolism.