How high is the hill and how steep is the slope? I am surprised the erosion control blanket didn't help, but I would think you will need something with more significant root systems than wildflowers to be successful.

It's 1:1 slope. I did install the blanket from the very top to the very bottom. Perhaps not enough staples is the problem; I used a staple for every 15-17" square. However, it shouldn't be such a big difference between 1' and 15-17." I am just so curious about why all the soil and mulch on the hilldsides on the roads I have seen all perfectly stay but mine does not. Catkim is right about root systems. In fact, I do see those plants on slope of the side roads being already mature or at lease much bigger than my seedlings. However, I am surprised to see those light weight mulch on those slopes not running down or blown away. Even the mulch on my flower beds has been blown everywhere

I asked about the height of the slope because I wanted to suggest digging flat areas to plant larger shrubs, but this may not be a good idea in a very big, very steep, unstable slope. There are many slopes in my neighborhood and I've observed these 'planting basins' dug into the slope. Let's see if I can create a diagram:

Slope profile->....../
........................../
.....................__ (planting basin)
.................../
................./
............__ (planting basin)
........../
........./

Perhaps I'll take a walk this weekend and photograph some of the methods used to deal with this problem, if you or anyone is interested.

Another good slope treatment is to terrace the slope as catkin describes.

An easy way to terrace a slope is to to trench in some coir logs. The logs trap sediment and seed behind them, creating terraces.

Where can I get long coir logs? I have seen a lot of places use the method. I was actually thinking of doing it too, but I just had no idea how to to do it and where to find long logs. Catkin and joepyweed, esthetically, do I do a one horizontal logs cross the entire hill (it's big though. I probably cannot find logs that long), or do I make sections, or do I stagger the sections, or do I make zigzag patterns? Catkin, if you don't mind, would you please take some pictures in your neighborhood? Thanks.. And would you please explain a little bit more about "planting basin" you were mentioning?

I'm thinking you could spread a layer of seeds and compost, put a little straw on top, than stake down a sheet of burlap with wooden stakes or branches. Water and light would make it in past the burlap and straw pretty easy. Plants would grow out through the mesh easily. Over time the burlap, straw and sticks would rot away to nothing. By then the plants should be established enough to stand on their own.

I finally got the photos I promised for you. This slope was recently planted out. Irrigation, erosion control netting, plants set in small basins, groundcover to finish.
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You can see how there is a small level spot where the plant sits so the water doesn't simply flow past the roots on the way downhill:
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Thank you so much Catkim for your promise and effort to help me. The hill looks nice, although there are probably more weeds as the greens than anything else. Everything balances out nicely between the green creeping stuff and the reddish-brownish spiky plant (what are they, by the way? Are they pretty good with clay soil?)

The problem of my hill is that it is not growing much at all even with green creeping weeds. It's mostly red clay look

In my experience, it's difficult to seed a hill for all the reasons that you've experienced. We tried and tried to grow grass and finally turned our short slope into a huge mixed border/flower garden.

Planting potted perennials, shrubs and ornamental grasses with roots will do more to hold the ground in place and survive. I create pockets to level out each spot as shown by Catkim's example. I know it sounds more expensive, but it's less likely to wash away and more likely to bloom and prosper for you in the long run.

I'm assuming that you have full sun for this site? If so, you've got a great place for a butterfly/hummingbird garden. The majority of those plants that I use are deer resistant and drought tolerant (once established). That mix is the closest that I can think of to give you the wildflower look.

Agastache
Asclepias tuberosa (drought) or incarnata (moist)
Echinacea
Crocosmia
lavender
Nepeta
rosemary
oregano
thyme
bronze fennel

Ornamental grasses like miscanthus and panicum are great.

buddleia 'Pink Delight' is a prolific bloomer and almost evergreen for me in zone 7, cut back in late winter
spirea 'Neon Flash' (leafs out early)
lantana 'Ham and Eggs' (has to be cut back in spring, so it's not real attractive over the winter)
caryopteris (like lantana)

Cameron

Here is a link that might be useful: lots of examples in my blog

Terracing is another option.

Thank you all. All good info. And Cameron, what a beautiful garden you have. I am so jealous!!!