Humble, Invaluable Lobularia maritima
The humble annual Lobularia maritima, common name "Sweet Alyssum" is my greatest unsung garden hero. It reseeds reliably and prolifically (buy one six-pack of this stuff at a big box store and you're set for life), but is easily pulled out at any stage of growth due to its tiny root system, which tends to remain tiny even when the plant itself is two feet (60 cm) across:
Native to the Mediterranean, it spouts here when the winter rains begin, creates a frothy white carpet of blooms for all of spring and a bit of early summer, then dries up by fall. I pull it out when it begins to look ratty in August. Irrigation brings up a few seedlings from summer through fall, and when the winter rains return, so does a grand new crop of L. maritima.
One of its great virtues is that it provides a constant supply of pollen for bees and beneficial insect predators such as Ladybugs and Lacewings. The pollen provides plentiful food for predatory insects when their basic food source, aphids, are not present. It's an ideal way to keep beneficials in the garden year-round until aphids make their appearance.
Another virtue is how long it will look good even without irrigation. And it's a great underplanting for roses, because L. maritima's wimpy root system won't out-compete the rose root system:
Plus, it's even pretty.