I live in zone 4b (Twin Cities) and I have a very shady back yard in a house I bought 2 1/2 years ago. The "yard" was mostly bare dirt, so I tried to reseed as a lawn, but 1) it's too shady and 2) the soil is lacking in Nitrogen, Phosphorus and Potash (I did a home soil test). At the least the Ph is balanced! I thought adding starter fertilizer would do it, but I've learned that that was not enough to permanently amend the soil. So I'm going to start fresh and just aim for some pretty ground cover that can handle heavy shade. I'm springing for 6 cubic yards of a 50-50 soil/compost to till into the soil (my yard is an odd shape, but it should be enough for at least 1 inch of new material on an area that's roughly 40 ft x 40 ft.). Given that expense, I want to be judicious in how much ground cover I purchase to spread over the area. How do I determine how many plants I have to buy, if I want the area to be filled in, say, 2 years? Also, am I better off buying ground cover seeds and growing my own seedlings, so I can have more starter plants? (I've only ever been successful with growing tomatoes from seeds, and even those were pretty poor producers.) Is there one or two varieties of shady groundcover that's known to spread more quickly? Thanks for any info you can offer!
As to how many plants you need, read the mature size/plant spacing info for the plant in question...if the info says that the mature plant covers 1 foot, then you would need 1 plant per square foot...with the plant being in the center of the square...If a mature plant covers 4 inches, you would need 9 plants per square foot.
If you plan to have paths or a seating area in your yard, don't bother with amending those areas...use your new soil in the areas you are going to actually plant in. You can use string, a garden hose, or cheap spray paint to mark out your plan and make it easier to gain a visual image of the layout.
Over time, you can turn your area into a lush shade garden. You can add paths and maybe a bench or bird bath for interest. Full shade gardens can be quite lovely.
Here are some links with ideas for you
Ajuga reptans would probably work for you. It comes in a variety of colors and averages about 6 inches in height. It is a relatively fast growing plant and blooms in full shade. Scroll down to Ajuga in this link for images
Whatever type of plants you get either as gifts, root cuttings bulbs or buy from garden store, you can split them up by removing some soil, look for stems that have roots attached and either cut or pul the plants apart with roots, then set out the old and new plants. in a couple of years time you will have many more lovely planted out areas.
To amend your soil, I would look out for as much animal manure you can lay you hands on, (usually given free) make sure it is well rotted, no smell tells you it has composted down into a good quality loom ? soil.
You either lay this on top of your soil over winter or dig it into the soil mixing it with the poor soil you already have, lay it about 2-3 inches deep as you dig the soil and you will have an immediate improvement, dont forget to remove any weeds, roots and rubbish as you dig.
Plants that enjoy shaded areas are as follows:
Cyclamen (bulbs or tuber)
Helleborus (christmas Rose)
Viola. Vinca. Polygonatum.
Lilium, (Some bulbs)
there are many, many more plants for shaded areas and these named would require the soil to be improved as I have suggested, some will grow anywhere, there are spring bulbs and aurumn plants that would suit but as your funds are low, I gave plants that you can gather over the year.
Best plantings show off best if the plants are planted in 3-5-7 etc, as a single plant of one type gets lost for ever or takes years to be large enough to show it's self.
Hope this helps get you started planning your layout in the shade.
Also have you a picture to show the area and we can see what's causing the dense shade, if it's tree's, maybe these can have branches removed to lift up the canopy allowing more light onto the area, if it's a fence /building or other structure then maybe we can help you better by seeing the problem.