i think longer term it's going to be very difficult to grow much under your conifers because their roots drain all the moisture from the soil, turn the surrounding area into a barren area and make it very difficult for plants to survive there. Additionally, the heavy shade and lack of light does not provide an encouraging environment for plants. You might be able to grow a few spring bulbs like bluebells or snowdrops which die off, later in Spring possibly some decorative ivy which seems to thrive in most difficult conditions but I suspect you will find it difficult to get plants thriving underneath them long term. Don't put in expensive plants as they prove to be a big disappointment and die after a short life.