Planting a hydrangea successfully is less about making an elaborate planting hole than about putting the right plant into a position where its roots can remain moist without becoming waterlogged. Light, drainage, shelter and mature size all matter before the spade goes into the ground, because a well-planted hydrangea is much easier to establish than one forced to cope with a dry, cramped or exposed site.

For the wider picture on choosing and caring for the genus, see our guide to growing hydrangeas in Ireland. This article concentrates on the planting itself: choosing the position, preparing the soil, allowing enough space and helping the shrub establish after planting.

Choose the position before you prepare the soil

Hydrangeas are often treated as though they all want the same conditions, but the main garden groups differ enough for position to be considered first. Mophead and lacecap Hydrangea macrophylla are happiest where the soil stays consistently moist and the plant is protected from the most drying conditions. In a cool maritime climate they can tolerate useful amounts of sun when moisture is reliable, although shelter from harsh afternoon exposure and persistent wind usually preserves the foliage and flowers better.

Hydrangea macrophylla ‘Ayesha’ is a useful example of this type: a substantial mophead suited to fertile, consistently moist soil in a sheltered border. Panicle hydrangeas are generally more comfortable in brighter positions, and Hydrangea paniculata ‘Limelight’ can be used where there is considerably more sun, provided the root zone does not dry out during summer. Hydrangea arborescens ‘Annabelle’, by contrast, benefits from reliable moisture and a sheltered position, particularly because its large flower heads can become heavy.

If the site is genuinely shady, position becomes a more detailed question than simply choosing the darkest corner. Our guide to hydrangeas in shade explains how the different types respond to lower light. Under established trees, root competition can be as important as shade itself; the broader guide to gardening under trees is useful where mature roots already occupy much of the soil.

Give hydrangeas moist soil, but do not confuse moisture with poor drainage

The most dependable soil for most hydrangeas is fertile, moisture-retentive and well drained. Organic matter is valuable because it improves the root zone and helps the soil hold moisture, while mulch reduces water loss from the surface. The important qualification is drainage. Hydrangeas may wilt quickly when they become dry, especially the large-leaved types, but roots held in saturated soil are also liable to fail.

On naturally good loam, preparation can be quite simple. Where the soil is poor, incorporate well-rotted compost or other mature organic matter through the planting area rather than treating the hole as an isolated pocket. Heavy soil needs more thought. If water sits around the planting position for long periods, improving the structure and, where necessary, raising the root zone is more useful than simply adding richer material to a wet hole.

Soil reaction matters most visibly with blue and pink Hydrangea macrophylla and H. serrata, but pH should not distract from the more basic requirements of healthy roots. A hydrangea in the desired soil reaction but poor drainage is still badly planted.

Use mature spread to decide hydrangea spacing

There is no useful single spacing for every hydrangea. A compact cultivar and a vigorous panicle hydrangea may look equally manageable in nursery pots, yet they can occupy very different amounts of ground when mature. The spacing decision should therefore start with the expected mature spread of the exact plant rather than with the size of the container in which it arrives.

Allow enough room for the shrub to reach its normal shape without blocking a path, covering a window or pressing immediately into neighbouring plants. This is particularly important with mopheads, many of which become broad, rounded shrubs, and with larger panicle hydrangeas. Pruning can modify size to a degree, but planting an inherently large hydrangea into a restricted space creates a permanent maintenance problem and may eventually interfere with flowering.

How to plant a container-grown hydrangea

1. Wet the root ball thoroughly. A dry pot can be difficult to re-wet once planted. Soak or water the container until the root ball is evenly moist, then allow excess water to drain.
2. Prepare a hole wider than the root ball. Loosen the surrounding soil so new roots can move outward. The final depth should allow the top of the root ball to sit level with the surrounding soil, or very slightly above it where drainage is a concern.
3. Remove the pot and inspect the roots. Healthy roots should be firm. If they are circling tightly around the outside of the root ball, loosen them gently rather than planting the root mass as an undisturbed coil.
4. Set the plant at the correct level. Do not bury the existing root ball deeply. Keep the top of the potting medium approximately level with the surrounding ground.
5. Backfill without hard compaction. Return the prepared soil around the roots and firm it sufficiently to remove large air spaces, but do not compress it into a dense layer around the plant.
6. Water thoroughly after planting. This settles soil around the roots and ensures that the original root ball and surrounding ground begin with an even supply of moisture.
7. Finish with mulch. A layer of about 5 cm helps conserve moisture and supports the cool, even root conditions hydrangeas prefer.

Do not make the planting hole richer than the ground around it

Hydrangeas respond well to organic matter, but soil preparation should encourage roots to move out into the surrounding ground. A small hole filled almost entirely with soft compost may look generous, yet the plant still has to establish beyond that pocket. Preparing a broader area where soil is poor gives the developing root system a more even transition into the garden soil.

Moderate feeding can be useful later, but it should not replace good soil preparation. Excessive fertiliser at planting is unnecessary where the soil already contains reasonable fertility and organic matter. Moisture, drainage and root contact with the surrounding soil matter more during establishment than trying to force rapid top growth.

Establishment depends on steady moisture

A newly planted hydrangea has not yet extended its roots far into the surrounding soil, so it is more vulnerable to drying than an established shrub. Keep the root zone consistently moist while new roots develop, checking the soil rather than relying only on the appearance of the foliage. Large-leaved hydrangeas can flag in warm conditions even when some moisture is present, so the condition of the soil is the better guide.

Water at ground level where possible. Directed watering reaches the root zone efficiently and avoids repeatedly wetting leaves and flowers. Mulch is especially useful after planting because it slows surface drying, but it cannot compensate for a root ball that was allowed to become dry before planting.

Exposure increases the demand for water. If the garden is coastal or regularly windy, the same hydrangea can establish more slowly than it would in a sheltered border because leaves and flowers lose moisture more quickly. The separate guide to hydrangeas for coastal and windy gardens deals with that siting problem in more detail.

Common hydrangea planting mistakes

• Choosing the plant before checking the site: sun, shade, wind, drainage and mature size should decide which hydrangea belongs there.
• Planting too deeply: keep the top of the root ball close to the surrounding soil level rather than burying the base of the shrub.
• Treating a wet site as ideal: hydrangeas need dependable moisture, but they still require oxygen around the roots and therefore good drainage.
• Ignoring circling roots: tightly confined roots should be loosened gently so they can move into the surrounding soil.
• Spacing from the nursery pot: use the plant's mature spread, not its current container size, to judge how much room it needs.
• Letting the root ball dry during establishment: newly planted shrubs need steady moisture until their roots have extended into the surrounding ground.

Planting hydrangeas in pots is a different job

Container-grown hydrangeas depend entirely on the volume of compost in the pot, so drainage, watering and feeding behave differently from plants in open ground. If the intended position is a patio, balcony or paved area, follow the separate guidance on growing hydrangeas in pots rather than applying garden-soil advice to a container.

Start with the site, then choose the hydrangea

The best planting work begins before digging. Match the hydrangea to the available light and shelter, make sure the soil can hold moisture without remaining saturated, and leave enough room for the shrub's mature spread. Once planted, keep the root zone evenly moist, mulch the surface and allow the plant to establish rather than forcing growth with heavy feeding.

If the site is suitable and the planting area is ready, you can browse the Hydrangea collection and choose a type whose mature size and growing requirements fit the position you have prepared.