Start with the "daisies" emitter you're talking about, since it's close to what you want.
- Add it to the stage.
- Open the emitter properties dialog.
- Click the "particles" tab, and check "single".
- Now select the "active" property in the hierarchy, and add a point in the graph at frame 100 or so, then adjust that until it's the way you want it.
- You may need to adjust the "size over life" graph too.
- On the "Change Shape" page of the particles page, select the leaf image (if it's in the library) or click the "new shape" button and load your own leaf image.