Answer: It isn't a good idea to add soil over the roots of your pine. The roots can suffocate if you change the soil grade. I know that the roots seem to be a single network and difficult to dig in and around but if you carefully explore with a sharp stick you'll be able to tell where the larger roots are and where the soil is deep enough to plant in. Once you locate the larger roots, you can plant in between the roots.
It's difficult to find plants that will grow beneath pines. The soil beneath pines tends to be on the acidic end of the pH scale because of the needles they drop. Another problem associated with growing plants under pines is that the trees have fibrous roots near the soil surface which will compete with any other plant for moisture and nutrients. Additionally, the canopy of pines effectively divert rainwater so things are usually pretty dry directly under the trees. Now that you have a little background information on why it's so difficult to grow plants under pines, you're better equipped to make the right plant choices. Look for plants that grow well in acidic soils, have shallow roots, grow in partially shady locations, and don't mind dry conditions. Some of the plants that will adjust to these conditions include Variegated Solomon?s Seal (Polygonatum multiflorum ?Variegatum?), Hardy Plumbago (Ceratostigma plumbaginoides), Creeping Oregon Grape (Mahonia repens), Lady?s Mantle ?Thriller? (Alchemilla mollis ?Thriller?), Anaphalis, Antennaria, Artemisia ludoviciana, Aster divaricatus, Bergenia, Brunnera, Campanula carpatica, Corydalis lutea, Eupatorium rugosum, Galium, Geranium maccrorhizum, Helleborus foetidus, Hemerocallis (daylilies), Heuchera, Iberis, Mertensia, Polygonatum, Pulmonaria, Thalictrum, Vinca minor and Viola.
I've had great success with vinca minor, daylilies and a groundcover called dead nettle. Hope these work for you as well.
Best wishes with your garden!
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