If I'm mulching, will plants self-seed and spread?

Hi, I just mulched a new bed of coneflowers. I am excited because these are native and I am looking forward to seeing them bloom (maybe next year, though I am seeig some buds) and hopefully attract birds in the fall!

I mulched because that is gardening goodness, as understand.

My question is exactly what's in the header. How do I get the plants to self-seed and spread if I have mulched them? I'd like my bed to fill out but I am not sure how this happens if the mulch will suppress the new growth.

I'm equally perplexed with Siberian Iris, though in my rather limited experience, iris would be less of a problem.

Mulches are normally good for the garden where there is bare soil, as the mulch breaks down to compost, the soil when dug is enriched , air is added, the composted mulch helps retsin moisture in the soil AND can act like a blanket to protect roots etc from frost / freeze should you get that in your area.
You never mentioned what you used as mulch or how deep the mulch is, that would give more info regarding did you do well or not LOL.
Humus rich mulches are best for using in flower / veg areas as when they break down into the soil they do as mentioned above.
IF you use gravel, large wood chips etc then these act as a weed preventative rather than add goodness to the soil for Veg / Flowering plants and are best used for pathways, around trees or any ground that's NOT in use but bare and needs weed prevention.

How to get self seeding to take place. Depending on the plants, nature can help this, even IF there's mulches, the mulch should be loose in structure enough to allow little seedlings to take hold.
You get weed seeds growing all over mulches so be assured that if the mulches are NOT thick wood, then self seeding should take place BUT be aware, birds, insects animals all help spread seeds weeds or otherwise, wind blows seeds around too.

I would think the best way for you to sow or get seedlings to grow in the patch is to pick the DRIED seed-heads from the type of plants you want to resow, either scatter them at the CORRECT time of year OR sow the seeds into a pot of good quality seed compost and germinate them inside or outside by placing the pots in a sheltered place. I would cover the pots with a sheet of glass or clear kitchen cling film you cover food with BUT make a few holes for ventilation, no air flow will bring mould to the seeds. maybe take the cover off in day time and replace for night. water the seeds in the pots from below by sitting the pots in a saucer of water till the soil goes darker, allow the pot to drain. by bottom watering you dont disturb the seeds.

Hope this helps you out a little
Good luck.
Kindest Regards.
WeeNel.

If you put a 'standard' 2" layer of mulch down, you will probably see volunteer seedlings starting up next spring. Mulch will not stop 100% of seeds from growing and you may see some weeds the first year. Also, a mulch layer will not stop plants from spreading under the soil. In my two main perennial beds, the only mulch that goes down is a light layer of shredded leaves and lawn clippings in the fall.