How to Care for Mature Apple Trees
To care for your mature apple tree, you'll need to prune it and fertilize the soil. Prune your apple tree in the late winter or early spring. This gives them enough time to recover before they start to bud in the spring. Cut off any broken, dead, or diseased branches. These might have a strange color or be losing their bark. You should also fertilize your tree's soil each spring. If it's grown less than 8 inches, use a general 10-10-10 fertilizer. If it's grown more than 8 inches, use a 0-0-50 fertilizer. Spread a pound of fertilizer in an even layer around the tree using a hand fertilizer spreader or a small trowel. This helps it saturate the soil evenly. Try to remove any fallen fruit from the ground as soon as it falls, which will deter pests from approaching the tree.
Complete your pruning in the late winter or early spring. Pruning your trees when they are dormant during the cool months gives them ample time to recover before they begin to bud in the spring. It also makes it less likely that pests will enter from the pruning cuts that you've made. Avoid pruning in the summer months, if at all possible. Remove any broken, dead, or diseased branches. Look over your tree and use hand pruners to cut away any smaller problem branches. For larger limbs, you may need to stand on a ladder and cut these away using lopping shears or a pruning saw. Target those branches that are off in color or appear to be cracking or losing their bark. Clip away any water sprouts. These are smaller shoots that split away from lateral branches. They usually grow directly upwards and can harm your apple trees of valuable sunlight and nutrients. Getting under the canopy of your tree and peering upwards will help you to spot these sprouts. If your tree has a large number of water sprouts, this could be a sign that you might have over-pruned in a prior season. Cut away tall branches at the top of the canopy. Ideally, your tallest riser branches should reach a maximum height of 18 feet (5.5m). Use a pole saw to safely remove these branches. Cutting out entire large branches from the upper half of the canopy allows light to penetrate into the tree's interior. Trim back any branches that bend towards the ground. As a general rule, you want your tree branches to grow almost parallel to the ground. As you are pruning, try to adjust those branches that are starting to head towards the ground. Also, make sure to keep an eye on straight vertical branches, too. Create thinning cuts by removing entire branches. Cut away full branches close to the trunk of the tree. If you cut a branch right around a bud, then you can actually inadvertently boost the growth of the remaining limb. This is another reason why it's always important to carefully consider before you even attempt to remove a branch to begin with. Make clean cuts. Whether you are removing an entire limb or just a branch section, sever it with a single slice when you can. This may mean that you'll need to put a bit of extra force behind your movement. Try to leave the collar intact as well. This is the small, bumpy edge that connects branches to other branches or the stump. Spread out your major pruning over 3-4 years. If you prune too heavily all at once, then you could encourage disease or poor fruit development from your tree. Instead, assess your pruning workload and split it out over a number of years. Trim back a few branches from the underside, sides, and top during each session. If you see a large number of water sprouts or if your apples are discolored, these are signs that you've over-pruned and may need to scale it back for future sessions. Test for a pH range of 6.2-6.5 in the spring. Contact the county extension or agricultural agency in your area for advice on pH testing. Many of these agencies can provide you with a sampling kit. Otherwise, order a kit online or purchase one at your local gardening store. An ideal pH for a mature apple tree should fall between 6.2-6.5. Skip fertilizing if your pH is ideal. If your pH results fall between 6.2-6.5, you may want to avoid fertilizing altogether. It's possible that changing the soil composition could change those pH results in a negative way. Keep testing each year in the spring to check the pH levels. Apply lime or sulfur to correct your soil’s pH. If the test results show that your soil is less than 6.2, then buy a package of lime from your garden store. If the test results show that your soil is more than 6.5, then buy some sulfur. Sprinkle the lime or sulfur across the ground under your tree, as directed by the product's packaging. The lime helps to correct overly acidic soil and the sulfur does the same thing for alkaline soil. You may have to apply the sulfur or lime multiple times before you see results. The packaging will tell you about application frequency and when/if you'll need to re-test. Rake any debris out from under the tree prior to fertilizing. Get a rake and remove any leaves, sticks, or fallen apples. As a general rule, do your best to keep leaves away from your tree as they can easily spread illnesses into the trunk. Creating a clean area also helps the fertilizer to absorb more effectively into the dirt. Choose your fertilizer type and amount based on the tree’s growth rate. If your tree has grown less than 8 inches over the past year, then you should apply 1 pound of a general 10-10-10 fertilizer. If your trees are posting more than 8 inches of growth, 1 pound of a sulfate-based 0-0-50 fertilizer is usually a better option. A 10-10-10 fertilizer contains 10% potassium, 10% phosphorous, and 10% nitrogen. The 10-10-10 fertilizer should be applied in early May, whereas the 0-0-50 fertilizer should be applied in early spring. Avoid fertilizing your trees after July 1st. Apply the fertilizer in an even layer. Using a hand fertilizer spreader or even a small trowel, spread the fertilizer under the tree from the trunk to the drip line. The drip line is where the spread of the branches end and rainwater can drip down. Double check that the fertilizer appears evenly applied, so that it saturates the soil evenly. Water every few weeks if needed. Most mature apple trees require very little water in order to survive. If you live in an area that is relatively dry, consider watering your trees once every 2-4 weeks for 30-60 minutes at a time. The key is to do a deep watering that reaches the roots underground. When you are watering, saturate the area from the base of the tree out to the drip zone area. Remove all dropped fruit immediately. Make a habit of policing and raking the space under your apple trees regularly when they are bearing fruit. Pick up and dispose of any fruit that has fallen off of the tree. This will help to minimize the chance that pests will be drawn to your trees by the tasty treats on the ground. Install mouse guards around the tree. You can either purchase metal or plastic guards from your local garden store. A mouse guard essentially circles around the base of the tree and blocks access for pests. To be fully effective, the guards should be at least 18 inches (46 cm) high with 1 inch (2.5 cm) or more buried within the ground. Check your guards regularly to ensure that they haven't stretched out or broken apart, which could provide access to mice or other rodents. Hang sticky traps from branches to kill insects. Look for traps designed specifically for trees or bushes at your local gardening center. Use string or wire to position 1-2 of these traps on each tree where you've noticed an infestation. Japanese beetles and other insects will be drawn to the sweet smell of the trap. Spray your trees with neem oil to kill off borer insects. It's best to prevent a borer infestation, which is difficult to detect and can easily kill your trees. Buy neem oil at your local gardening store and spray it liberally over the trunks of your trees. Repeat this process when your tree's buds are green, when they turn pink, and after their petals fall away. You can also purchase chemical pesticides that contain neem oil, such as Neemix. If you intend to eat your apples, avoid using toxic or chemical-laden pesticides. Always wash your apples before eating them. 
Complete your pruning in the late winter or early spring. Remove any broken, dead, or diseased branches. Clip away any water sprouts. Cut away tall branches at the top of the canopy. Trim back any branches that bend towards the ground. Create thinning cuts by removing entire branches. Make clean cuts. Spread out your major pruning over 3-4 years. Test for a pH range of 6.2-6.5 in the spring. Skip fertilizing if your pH is ideal. Apply lime or sulfur to correct your soil’s pH. Rake any debris out from under the tree prior to fertilizing. Choose your fertilizer type and amount based on the tree’s growth rate. Apply the fertilizer in an even layer. Water every few weeks if needed. Remove all dropped fruit immediately. Install mouse guards around the tree. Hang sticky traps from branches to kill insects. Spray your trees with neem oil to kill off borer insects. 