You don't need to spend Quintessence, unless the task you want to do is too difficult (usually):
Quintessence does make working magic easier. As the mage reinforces his spell with the power of reality, it bends the cosmos more smoothly. Each Tradition has its own views on what, exactly, Quintessence is and why it serves as a power source like this. The upshot is that each point of Quintessence channeled into an Effect lowers the Effect’s difficulty by one point, to a maximum net difficulty modifier of three – so you can cancel some penalties and perhaps lower the difficulty by as much as three points.Source - heree is a handy table for difficulty levels which also confirms that
With Life 2 you can change the plants, this means that you can make them grow, change shape or type. So you could make the flowers to bloom suddenly which would be a coincidental, easy and safe type of magick or for example, make the flowers much bigger or change into another type of plant - small bushes, poison vines etc, which would be a vulgar magick. The latter one is much more difficult (hence dangerous), so your mage might want to spend the quintessence to reduce the risk.
If you'd like to do something even more spectacular, like turning a daisy into a tree, your GM would probably decide that all that plant mass cannot get from the daisy itself, so you'd have to add the Prime sphere to channel the loose Quintessence from the "thin air" (not one stored in you!) into creating the tree using Prime 2
Prime 2: Create Pattern: In conjecture with other Spheres, the mage can create a simple Pattern out of thin air, by diverging the flow of Quintessence in the Tellurian into the new form he wants to create. source
Again, you could spend your own Quintessence to make this extremely vulgar effect easier.
If instead of a regular tree you want to make a magical tree (which for example bears fruits that help healing), even that might not be enough and at least Prime 3 might be required:
Prime 3: Channel Quintessence: The mage can draw free-flowing Quintessence from a Node, Juncture or Tass into their pattern or transfer Quintessence from their own Pattern into another Pattern. ibid
This time you would be indeed using your own Quintessence not only to make the task easier but to make it possible in the first place.
Edit: As @Cornerfield pointed out, if you don't have enough skills in Prime, you can spend your own Quintessence to create "something from nothing" (like in the second example above)