short with flowers
No evidence has proven that alien races exist, but if they do they may have plant-based characteristics (i.e. trees, flowers, cacti, etc.).
because there is only a short period of time when the plant is in flower and some plants produce different colored flowers
No, flowers are a parts of a plant. Some plants don't have flowers, but all flowers come from plants.
no some oak trees have flowers on them
Umbrella plant, urginea and ursinia are flowers. They begin with the letter U.
It is because of it's genetic. Every type animals and plants has it's own unique combination of genetic. So some flowers are tall and some short due to this reason.
there color and smell is diffrent from certain other flowers.
Some characteristics of lilacs include long, cone-shaped panicles that are covered in tubular tiny flowers and and flagship purple flowers. You can learn more about this flower at the Wikipedia.
Venus fly trap
Producing flowers costs the plant in terms of diverting resources. If, for example, the insects which are likely to pollinate the flowers are not around at that time of the year, the production of flowers would be an expensive but pointless activity.
You should recycle and plant some flowers for the hummingbirds and bees.
Flowers, weeds, any vascular plant.