Most professionals recommend an annual application of pine straw. However, if you're only using it for decorative purposes, you can apply it about twice a year to keep the landscape looking fresh.
You may want to wear a pair of gloves when applying pine straw. The needles can be very prickly and harder to manage with your bare hands. Application is quite simple. Remove the bale ties and simply take handfuls of the straw, shaking it over the application area. In general, plan to apply to a depth of about 3 inches, adding approximately an inch annually to maintain a good depth.
The main goal is to prevent weed growth and enhance the beauty of your yard with ground cover. Your application of pine straw needs to be at least 3 inches thick. You may find recommendations for application up to 6 inches thick, but that is usually not necessary, especially in shady locations such as areas beneath trees. This decorative approach is a nice technique to use for lining walkways and seating areas that aren't paved, such as the areas around benches.
Gardens, Trees and Shrubs
It's a good idea to extend the pine straw to the drip line of your plants. Keep the straw about 2 to 3 inches away from plant bases and the trunks of trees and shrubs. Pine straw applied in this way around trees will discourage rodents from feasting on the bark.
As pine needles break down, they slightly acidify the soil, making them an excellent landscaping mulch for acid-loving plants, trees and shrubs such as camellias, azaleas, hydrangeas, fuchsias, gardenias, ferns, dogwoods, magnolias, holly and evergreens.
(copied from lowes.com)