I used to run a landscaping business in NorCal and I planted a lot of CA native gardens. In general you wouldn't amend the soil for native plants, but that's assuming you're planting locally native plants in the same kind of soil and habitat they would normally be growing in. And that your soil is relatively healthy. Stripped, graded, compacted soil from recent construction will need some kind of work before you could plant anything except stuff that likes those conditions. California is a political designation, plants don't care. There are multiple ecoregions within that political boundary. Plants native to one, won't necessarily grow that well in others. I had a real issue with that in my landscaping. Most of the 'CA Native' plants available from nurseries were from particular ecoregions, like the Channel Islands. They did ok most of the time but they weren't frost tolerant at all. Bush monkey flowers (Diplacus) were a popular native plant when I was working there, but they barely made it through most SF Bay Area winters and were all wiped out in one bad winter. Native to CA, but not locally native to the SF area. Then even within a particular ecoregion you have different habitats. Big-leaf maple is a CA native but it's a riparian tree, needs water and drainage. Plant it in the central valley or the desert and it's not gonna make it on its own. Up here in Washington it's a weed, grows anywhere and everywhere. Even the driest soils are wet enough for it. Down in CA you need a wetter soil for it to thrive. But not a wetland soil. I would plant stuff in unamended soil usually, with drip irrigation for the first couple years or so, to get them established. Then depending on the species and the weather they ideally wouldn't need irrigation after that. Or fertilizer. But that's assuming you're choosing locally adapted native plants. Woodland plants would like mulching, but desert or alpine plants probably not. I don't believe I mulched chaparral plants, like the manzanitas and ceanothus. Chaparral soils tend to be dry and lean and that's what the plants native there need. Rott: pepper trees are not native to CA. They're from South America, even though they're called CA Pepper Tree. And I'd be careful with Matillija poppy. they're a garden thug. Unbelievably aggressive rhizomes. I wouldn't give them any compost or fertilizer or water, that's for sure!

I wish I could have gone since I'm very interested in using native plants.

As a compromise, you can choose to leave a few plants (at least 2 or 3) until the seeds ripen. Even one plant can produce a lot of seeds, but the more plants you can leave, the better genetic diversity you'll get. Think about it as a birdseed garden. I have calif. poppies and globe gilia and a few clarkias. I pull up the poppies when I need the garden space to put something else in, because I know there are enough seeds in the soil already. I mulch my garden, and the tiny globe gilia seeds reseed themselves prolifically anyway. I collect a huge amount of seeds each year, let the birds have as many as they want, and pull out the globe gilia plants when they're crowding other plants, and they're slowly taking over and spreading from one corner of the garden. I've found I can replant the pulled-up plants easily, and I've given away a lot of plants. They're in bloom much of the year because I water other plants in that part of the garden. The clarkias are not as prolific, but they come up in the garlic patch and I disturb the soil a lot in that part of the garden, adding compost and mulch throughout the year, so it's surprising they manage to come up at all! I pull up the garlic as I need it, and by the time the clarkias are big enough to bloom, I will have harvested most of the garlic. Most years I think about saving some seed, but I'm busy in other parts of the garden by the time the clarkia seeds drop, so I haven't saved any yet. I also have some of the delicate ground-hugging Clarkia davyi, which reseeded itself in a few containers from the one plant I had last year, so I'm planning to move some of the containers to another part of the garden when they start going to seed. By the way, that's the easiest way I know to grow wildflowers: put a container of the flowers in the spot where you want them to grow the next year, let them go to seed, and they'll reseed themselves there. I've found that I mostly give away any seeds I collect, since so many of these reseed themselves. I don't know if the phacelia tolerates transplanting, but if you were going to pull them out anyway, you could try transplanting a few to the area where you want a meadow and let them go to seed there. Just be sure to water them and keep an eye on them for a few days so they don't die before ripening the seed. The most important thing about planting a meadow is to make sure the area is as weed-free as you can get it before starting, so that weeds won't outcompete the wildflowers. Or, if you've learned to recognize young wildflower seedlings, you can carefully weed out anything else while all the plants are small.

Anytime can theoretically work, but it's definitely more difficult during the dry season. Many natives are very sensitive to transplanting stress, which is greater if you're transplanting them in the summer heat. If you plant any natives in the summer, I recommend planting them at night, so the roots aren't exposed to quite as much heat during the transplanting process. Planting them during the very end of the wet season can also cause problems, because the plants may not have time to get their roots out before the summer drought sets in. I planted a silver bush lupine this past March, and it grew a foot, produced a flower spike, then shriveled up in May and hasn't recovered, despite the fact that I'm now watering it considerably more than before. (It's not actually dead so far, but it looks terrible, and I don't know whether it will survive or not.) My other silver bush lupine, planted in January, is still perfectly healthy - probably because it had more time to get its roots out before the summer. A lot of natives are also prone to die from overwatering, especially in the summer, so I think it's much safer to try to plant natives early in the wet season and let them get their roots out before summer, rather than planning to give them extra water during the summer to keep them alive when they're newly planted.