When you're about to plant, make sure the plant's root ball and surrounding soil are well-watered. If your plant has a dry root ball, you can water it all you want, but water will never pass from the surrounding soil into the root ball.
Here's how to avoid this problem:
- Soak the root ball. Before you plant, fill a 5- gallon bucket with water, then submerge your plant (still in its nursery pot) in the water until it's fully saturated. You'll know it's saturated when you stop seeing bubbles percolating up from the plant's roots.
- Soak the hole. Place your now-wet plant in the planting hole, then fill the hole with water and allow it to drain. Repeat if it drains quickly, or if it's hot outside.
- Fill in the hole. After the excess water drains away, you can "firm in" the hole around your new plant with more soil.
Bonus Tip #1: For larger plants that won't fit in a five-gallon bucket, just make sure that you've thoroughly watered the plant in its nursery pot. (Soak the hole as you would for a smaller plant.)
Bonus Tip #2: Don't make a "mud pie" when you soak the planting hole. That ruins your soil structure. It's like Goldilocks: the soil should be not too wet, not too dry, but juuuuust right!