Some seeds need the cold stratification, like most milk weeds. Others do not, like zinnias. Select the seed you want and do a search for their specific germination needs.
Also some seed suppliers pre stratify. They usually don't tell you on the package. For those I just go ahead and plant in the fall or winter. It does not seem to hurt them.

Plants are divided into these groups:

Perennials (those that return in return in the same spot you plant in year after year)

Annuals (those that complete their life cycle in one year) and include:

Hardy annuals thrive in cool weather and are usually planted in the fall.

Half-hardy annuals can endure light frost; they are generally planted in early spring.

Tender annuals die in freezing temperatures and are planted in the spring.

Zinnias are tender annuals, and seeds should be planted after the last frost in your zone.

Most, but not all, perennial seeds need a period of vernalization, or cold stratification, in order to break the seeds' dormancy. Most milkweeds grown in the U.S. are native to the U.S. and are of this type. Other the milkweeds may be perennial in warmer or more tropical zones, don't need to be stratified, and are grown as annuals from zone 8, sometimes 7, north (depending on our winters), including Tropical Milkweed (Asclepias curassavica), Oscar Milkweed (Asclepias physocarpa), and a few others. In your zone, these may grow year-round.

Asclepias fasicularis or Narrow-leaved milkweed, is a perennial milkweed native to the Western U.S. I hope someone from that area will come in an help us out on that milkweed in particular. I don't grow it in my garden because it is a Western native.

Perennial milkweeds usually require at least 30 days of cold stratification. You can "winter sow" them - check out the Winter Sowing Forum here; you can stratify them in a baggy containing 1/4 cup of sand, 1 tsp water, in the fridge), remove them after 30 days minimal, and plant them in pots. Sandy is quite knowledgeable on this method and she can help us out in this regard. I usually winter sow mine recycled 2 litre soda bottles.

Perennials, from seed, usually follow this pattern: 1st year they sleep, 2nd year they creep, and 3 year they leap. So it can be 3 (2 at a minimum) years before you see significant growth and flowering in perennial milkweeds.

Other perennial milkweeds you might want to consider growing in your area include six that are commercially available (some harder to find than others), Asclepias californica, A. speciosa (I grow this one as it is native in Oklahoma as well), A. fasicularis (Narrow-leaved Milkweed), A. cordifolia (Heart-Leaf Milkweed), A. vestita (Wooly Milkweed), A. eriocarpa (Woolypod Milkweed). That said, A. speciosa and A. fascularis are the most common that I see folks growing on this forum. I think that A. cordifolia is a beautiful milkweed, in particular, with its purple flowers. I have seen seeds offered for sale of all of these milkweeds, though. Nine other milkweeds, for a total of fifteen, are native to California, but only these six are generally found to be for sale (seeds and/or plants).

If you have a native plant nursery in your area, or know of native plant societies or groups in your area of California that may have annual plant sales - I know it's a huge state - you might try to find out where you can locate plants, if you are interested. Plants will obviously give you a jump start over seeds.

Those warmer climate milkweeds like the Tropical, Oscar, or Giant Milkweeds will grow quickly and bloom for you in one season. You can collect seeds to keep your collection going of these.

In your zone 9, I'm not sure what temps you're experiencing now, as to whether you can start seeds now outdoors.

I'm hoping someone else can supplement this post that has experience gardening in California in your zone. Larry_Gene may have some additional info for you.

Try googling "Butterfly Encounters milkweed seeds" and check out the milkweeds they offer. They are an excellent source of viable seeds.

In regard to Zinnias, most are attractive to butterflies. I prefer sticking to the single flowering varieties that afford a flat landing pad for the butterfly. Some I am looking at for planting this year include the Whirligigs/Carousel, Chippendale, Profusion singles, Cut & Come Again mix, Zahara series, Sombrero, Old Mexico. Some of these are mixes that will produce both single and double flowers. If you go to the Annuals forum, there is a lot of discussion on various types of Zinnias on the market.

I am going to order a lot from Hazzard's Seeds (go to their site and use the search function for "Zinnia" and you'll get a listing of all their Zinnias). They are wholesale, but will sell retail. For instance, I can get 1,000 Whirligig's for $6.55. I can 1,000 of the Sombreros for $5.10. That is CHEAP! Some of the seeds are more expensive, but there are a lot of varieties you can buy in large numbers for very, very reasonable prices compared to the seed numbers you get from other places.

Have fun!

Susan

Here is a link that might be useful: Milkweed Seeds

If you want big plants - buy them at the nursery, but except to pay a lot.
You can try Asclepias curassavica - it will do well in one season.

I'ved started about a dozen Asclepias species from seed, most of which are native to eastern North America and may not be the species that grow on the West coast. Although I do have a couple of 2 year old Asclepias speciosa plants. Generally speaking in my experience -

- the Asclepias species that are perennial, have tuberous roots, and grow in upland (drier) habitat are slow to grow from seed. This includes A. syriaca, A purpurascens, A. variegata, and A. speciosa.

- A .incarnata (swamp milkweed) is a perennial wetland species, and grows much more quickly, will bloom the 2nd year, maybe even the first

- A. curassavica (tropical) is a fast-growing annual that might be perennials in your zone

As for Zinnias, I grow a large patch of the tall Zinnias every year - a mix of Cut & Come Again, State Fair, California Giants, or similar varieties. They are mostly singles and semi-doubles, which are more accessible for the butterflies, but I've read that Benary Giants which are fully double are attractive too. Tall zinnias are very popular with the butterflies, bees, and hummingbirds. I direct sow them when the soil warms up.

Here's a pic from this past fall that shows the Zinnias along with other annuals. Everything in this pic is started from seed!
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Terrene - that's a beautiful Zinnia patch! Have you ever tried the cactus flowering types? I just wondered. Altho they look semi-double to me, the central disk flowers appear more exposed on these than on the huge doubles like Benary and California Giants, for example.

Select Seeds catalog for 2012 has just come out and they are offering several Zinnias that I just love, one in particular being 'Jazzy Mix'. I attached a link to these. They are gorgeous! Pack of 100 seeds for $2.99 doesn't seem too bad either.

Susan

Here is a link that might be useful: Zinnia 'Jazzy Mix'

For me, Zinnias seem to grow better if I sow the seeds where I want them, rather than buying plants or starting them in pots.
Some zinnias do work better than others for attracting butterflies. See the ring of yellow things on the flower below?
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Those contain the nectar and some double zinnias don't have them or have very few. I think they're always yellow, no matter what the color of the zinnia, so you may be able to tell when you're looking at the picture of the zinnias on the front of the packet or in catalogs whether they have those. Also the very low growing types don't seem to attract butterflies quite as well as the taller ones.

Tropical Milkweed grows fairly fast from seed, fast enough that it's sold as an annual here. I think it might come back every year for you though. I don't know how hardy it is. It has colorful flowers and Monarchs love it for a host plant.

Hi Susan, I have never tried the cactus type. The main thing I do with the Zinnias is plant them in a new spot every year. Whenever I've done that, they have not gotten mildew. The one year I sowed them in the same spot as the previous year, they got terrible powedery mildew. Coincidence? Not really sure.

Christie, I love the tropical milkweed too and there are actually patches of it in the picture above in between the Melampodium. However, by the time I took that photo, most of plants were cut down and used as larval food for the Monarchs I was raising in August and September. And the few that flowered kind of got lost in the Zinnias. :)

The Tithonia in the background grew fabulous last year, then it got clobbered by Tropical storm Irene, however I staked it up and it made a nice come back. It grew HUGE! It is much loved by the pollinators too.

These annuals tend to grow pretty large though. Brieninsac you might need to grow a bigger butterfly garden!

All good information! I think I will try and find a milkweed at a nursery since I want something more established this year.

As for the Zinnias, is there a type that butterflies don't like? There seems to be so many seeds I don't want to buy one and find out they're not going to be liked.

Also, would you plant the Zinnia seeds in the spring or now? And will they flower by the summer?

Thanks!