Planting ephemerels and ferns straight into suburban and urban soil with leaves spread between them is not enough to keep them healthy and alive. Your site should be prepared well in advance of plants being put into the soil by adding organic material over a period of time.
Whereas with other native plants, one year of prep time is ideal, woodland gardens could take longer depending on your soil. You can often have your soil tested by your local extension office or there are mail-in kits you can buy from online companies who will analyze your soil. With these kits, you will be instructed how to collect soil samples to be tested. Most of the time, the recommendations that come back will involve adding chemical fertilizers to your soil, if it is deficient, but those quick “vitamin pills” are not what woodland plants need. Still, these tests can tell you about the content of organic matter and will let you know of the deficiencies of the soil as well as the make up of your soil in terms of clay, silt and sand. This is important since there are certain plants that cannot be grown in high clay content soil.
In general, to prep a site that is less than desireable, you will want to sheet mulch the area, laying cardboard with the tape removed, then covering that with a thick layer of leaves (or mulch). Let it stand to allow the breakdown of these materials. Keep a pile of fall leaves that you can continue to add to the area. Hopefully you can let this sit for 1-2 years at a bare minimum.
Understandably, this can be a disappointment when you are hoping to add some of these woodland jewels to your garden. Some of our plants are more adapted to lower quality soils and they can be planted either in the area or nearby as you wait for your better soil food web to develop. You can see a list of those plants HERE. Additionally, consider planting some woodland sedges and grasses. These are naturally some of the first plants to establish after a disturbance has occurred. They are quite important as soil erosion control, and their fibrous roots aid in developing a healthy soil food web.