Answer: Magnolias can take their time to come into bloom. Your tree is not in a hurry and may decide to take several more years. If you want to hurry it along and take a bit of a chance, you can prune some roots by taking a shovel and cutting down in 4 places around the tree about 1-2 feet from the base. This can shock a tree into early bloom but is also very stressful. Make sure it doesn't lack for water for a few months after the root pruning.
If it were mine I think I'd let it take its time. It will grow out of the juvenile state it is in and move into a reproductive state soon enough.
Your hydrangeas need a good mulch and plenty of water. About 4 inches of leaves or pine needles works great. These bushes are not very tolerant of drought, so plan on watering them twice a week, and more if they start to wilt. They may need even more shade than they are getting.
You can fertilize them some through the season. Compost is a good balanced source of nutrients, and there are many others which will do fine for hydrangea. Just don't push them too hard with fertilizer. Watch for yellowing leaves on the end of branches, a sign of iron deficiency. If you see this, it can be remedied with iron chelates (follow the label) and sulfur to lower the pH (1 pound per 100 square feet is a good place to start).
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