I have a cedar hedge which is well established and was wondering if Miracle-gro 24-8-16 would be ok to use?
Fertilizing an established cedar hedge in the spring can be a good idea even though it may not really be necessary. Since the root system of the cedars reach several feet from the actual hedge, using an organic compost spread several feet around the hedge will supply most of the nutrients that the cedars need. Applying a mulch of shredded bark or wood chips on top of the compost will help to retain moisture in the soil and to insulate the roots from temperature fluctuations. Keep the mulch a couple inches or so away from the trunks and spread it 3-4 inches thick to just beyond the branches.
However, if the needle growth is somewhat sparse and the colour is not as green and rich-looking as it should be, you could use the water-soluble Miracle-Gro 24-8-16; it is important that the first number (N-nitrogen) be the highest [N-K-P] to provide good needle and general plant growth. Make sure that you apply it gradually so that it will soak into the soil slowly; if you have sandy soil, you will need to fertilize more frequently for a while–fertilizer should not be used after mid-July.
Using a granular form of fertilizer may be more cost-effective in the long run. Simply spread the appropriate amount of fertilizer on top of the mulch and water it in thoroughly so that the roots will be able to absorb the nutrients.
Your hedge should remain healthy with the use of compost and mulch; however, an occasional dose of fertilizer may help to sustain the healthy growth of the cedar hedge.
Additional reference: https://www.extension.umn.edu/garden/yard-garden/trees-shrubs/fertilizing-evergreens-conifers/