Answer: It's really difficult to diagnose a plant problem without seeing it and having the opportunity to inspect the soil and the growing environment and the plant itself. Having said that, let's do a little detective work and maybe you can figure out what's happening with your pines. The first question to ask yourself is, aside from the browning needles towards the center of the plant, does the tree look otherwise healthy? Is it putting out healthy new growth? If so, the watering practices and the browning needles are separate issues. If the new growth looks sickly, lets concentrate on your watering. But first - here's something else to consider. Although evergreens remain green all year around, the oldest foliage, in this case the innermost needles, will eventually be shed. This is normal and natural. Needles usually last 3-4 years before they are shed. Now, back to the diagnosis. Check the inner parts of your pines for evidence of spidermites. These tiny little pests can cause yellowing and browning of the needles. Take a white index card or a folded sheet of white paper and hold it inside the branches toward the center of your tree and sharply tap on a branch or two. Some debris should fall onto the paper. If it moves, your tree has a population of spider mites. They look like pepper, but they move. If no spider mites are present, the inner browning is probably natural.
Now for the watering. There's a potential for each tree to get 8 gallons of water a day. This may be excessive, unless your trees are large. If they came out of 1-5 gallon containers, it's too much water. Cut it back by half by either plugging half the emitters or running your system for half as much time. If they came out of larger containers, it's probably an okay amount of water. Another consideration is the soil. If it's sandy, water will drain quickly so you won't have to worry about root rot no matter how much water you give your trees. If it is clay, the roots can suffocate because clay holds lots of water and whenever water saturates the soil and remains for long periods of time, it drives out oxygen which can suffocate roots. If you're not sure about the soil moisture content, dig down 3-4 inches and see how wet it is. You can adjust your drip system and do a series of checks until you know just how often and for how long your system needs to be on in order to provide enough water but no so much that the roots of your plants remain soggy.
Hope this information helps you figure out what's going on with your pines.
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