Hi, 

In general you want to space your plants to their maximum growing width on center to each other. However, there are some native plant designers that use a rule of thumb of plants being spaced about 12 inches apart in a grid like pattern or matrix. As the plants grow and fill in, they self edit, or you can actively remove some to let others grow. 

We can give you the rough plant width to space your plants that you have listed. Some of these species will spread more quickly than others so you may want to return to the space to clear out some plants from being more aggressive so you can have some input to the design of the garden bed and a variety of species. 

• obedience plant - aggressive spreader- you can space these out or plant in groups of 3 about 18-24 inches apart because they will fill in. Easy to pull if it starts to take over too much.
• sundrops - spreads - space out or groups of 3 about 24 -36 inches apart
• purple coneflower- reseeds- space at 18-24 inches apart
• wild bergamot- spreads- space 24-36 inches
• bluestem goldenrod - better behaved and forms clumps- space 18-24 inches in clumps or 24-36 inches from other plants
• woodland sunflower-vigorous spreader by rhizomes - space out through bed and give other plants about 24-36 inches clearance. 
  
  For the ground cover/living or green mulch: 
• hepatica, - this can be difficult to grow with out a low pH and fertile soil. It tends to prefer cooler weather.
• stolonifera phlox- space 12-18 inches on center to each other or weave through out planting space up to fill in between plants
• packera - space 18-24 inches on center or 24 inches from plants- more aggressive than the phlox
• wintergreen - this can be difficult to grow with out a low pH and fertile soil. It tends to prefer cooler weather.

Let us know if you have further questions. 

Emily 

Thank you, this is incredibly helpful! To clarify, did you intend the "groups of 3" to apply to all the non-ground-cover plants (so are purple coneflower, monarda, bluestem goldenrod, and woodland sunflower best planted in clumps of 3 plants together, or are those best planted individually)? Thanks!

Plant spacing and arrangement can be more art than science in some cases, based on the preferences of the designer or gardener. In my opinion, plants that tend to spread into larger clumps on their own (Monarda being a prime example) can probably be spaced-out solo with regard to their expected mature size (to accommodate that spread without them running-over less-vigorous neighbors). Plants that are more clumping and self-contained (coneflower, for instance) may benefit from being grouped (3 or more individuals near each other) in order to make more of a visual impact since each plant doesn't spread very far. This is not a set rule, though, and I have also seen spreading plants installed in drifts (swaths of multiple individuals of the same plant species/variety), where they still make quite a visual impact being en masse.

Therefore, you can do groups of 3 (or more) for all non-ground-covering species if you prefer, or just pick some to place in clusters while others stay solo and weave their way through the planting as a whole as they fill in with maturity.

We have a correction on the Hepatica conditions: it actually does not require a low pH (strong acidity), instead preferring a near-neutral pH, and also tolerates mildly alkaline soils well.

Miri

You're welcome!