Answer: Clay soil often doesn't drain well, so make sure you aren't overwatering as hydrangeas like well-drained soil. Roots sitting in constantly wet soil don't receive sufficient oxygen, and the plant won't thrive. The large hole is helpful, and adding compost to the backfill (for future reference) can also improve drainage. Layer compost or other organic mulch around the plant. The organic matter will improve the soil's
drainage and provide nutrients over time.
Water should soak about 2 feet deep for shrubs. To determine how far water has penetrated, poke a soil probe (any long metal rod) into the soil. It will move easily through moist soil, stopping abruptly where soil is dry. How often to apply water depends on rainfall, weather, how fast the soil dries, etc. but allow the top few inches to dry out before reapplying, or if the lower reaches seem really wet, wait longer. Also, you may be overfertilizing. Most landscape shrubs don't need such frequent applications (although products will tell you to apply often). Use a fertilizer in the spring just as new growth starts and then hold off for a month or so. If the plant looks like it needs fertilizer, reapply. The mulch layer will provide nutrients as well. If temps. are really warm, fertilizing stresses a plant, "forcing" it to grow, when it is better of conserving its resources, so fertilizing in the midst of summer isn't usually a great idea.
Sometimes it takes a while to get hydrangea to flower well. It can take several years. Hydrangeas prefer full sun or partial shade. If they don't bloom after the third season, consider moving them to a more appropriate site. Of course, moving them will disrupt flower production too, because transplanting disturbs the roots and sets the plant back. It may take a few years to recover but the plant should eventually bloom. I wouldn't recommend moving the plant again so soon.
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