Hydrangeas are popular plants in home landscapes, celebrated for their large, striking blooms. Achieving success with these shrubs begins with the initial planting process. Correctly preparing the ground, particularly the dimensions and shape of the hole, is fundamental to the long-term health of the plant. This preparation ensures the roots have the necessary space to establish a robust system.
Assessing the Root Ball
The proper dimensions for the planting hole are determined by the size of the hydrangea’s root ball. The first measurement to establish is the depth of the root ball, which is the distance from the base of the stem (the root flare) to the bottom of the soil mass. The planting hole should be dug to this depth or slightly shallower, but never deeper, to prevent the plant from settling too low.
The width of the hole is more important than the depth for encouraging outward root growth into the surrounding native soil. Dig the hole at least two to three times wider than the diameter of the root ball itself. For example, a 12-inch wide root ball requires a hole measuring 24 to 36 inches across. This provides a substantial zone of loosened soil where new roots can easily expand and establish themselves.
Digging the Hole and Soil Preparation
Once the size is determined, digging should focus on creating a profile that supports root expansion. The sides of the hole should gently slope outward from the base to the top, forming a shallow bowl or saucer shape. This shape encourages roots to grow horizontally into the surrounding soil rather than circling within the confines of the hole.
Roughen the sides of the hole with a shovel or garden fork after digging, especially in heavy clay soils. When a shovel is used in wet or clay soil, the sides can become slick and compacted, a phenomenon known as glazing. This glazed layer acts like a barrier, inhibiting water penetration and making it difficult for new roots to break through into the native soil.
The soil removed from the hole is used as the backfill, and heavy amendment is generally discouraged. Excessive amendment can create a pocket of rich, soft soil that causes the roots to remain within the hole rather than pushing outward, which is known as the “flowerpot effect.” If the native soil is poor, mixing a small amount of organic matter, such as compost, into the backfill soil is acceptable, but the mixture should still be predominantly native soil. Loosening the soil at the bottom of the hole slightly with a fork may aid initial root penetration and drainage.
Final Placement and Backfilling
Proper planting height is a common area where mistakes occur and can lead to health issues for the hydrangea. The plant must be placed so that the root flare—the point where the stem widens before the roots begin—is positioned slightly above the surrounding grade. Planting the root ball one to two inches higher than the surrounding soil level allows for expected settling and prevents water from pooling against the stem, which can cause crown rot.
The backfill soil is gently shoveled around the root ball and tamped lightly to eliminate air pockets. Avoid compressing the soil too firmly with your feet, as this can compact the soil and restrict oxygen flow to the roots. Once the hole is filled, the surrounding soil can be feathered up to the slightly raised root ball, creating a gradual transition. A thorough watering settles the soil and provides the moisture needed for the hydrangea to begin establishing its root system.