How to Plant Cranberries: Soil, Acidity and Bed Preparation

The flooded bog is a harvest method, not a growing requirement

Every photograph of cranberry growing shows people in waders pushing a red raft of floating fruit across knee-deep water. That flooding happens for a few days at harvest and again for winter protection on commercial farms. It is not how the plant lives for the rest of the year. A cranberry bed is drained for the whole growing season, and the vines spend spring and summer on damp ground with air around the roots, not underwater.

So the home version of a cranberry bed is not a pond. It is ground that stays reliably moist, never dries out, drains freely enough that water does not sit stagnant, and is strongly acid. Standing water through summer suffocates the roots and rots the runners exactly as it would with any other shrub. If you build a sealed waterproof pit and keep it full, the plants sulk and die within two seasons.

The rare exception is winter flooding in very cold regions, where a shallow sheet of water freezes over the vines and protects them from drying winds. Even then the water goes on in late autumn and comes off before growth starts.

Acidity is not negotiable

Cranberry belongs to the same family as blueberry and carries the same root chemistry. It has almost no root hairs, depends on fungal partners in acid peat, and takes nitrogen up as ammonium. The target range is a pH of 4.0 to 5.5, which is lower than almost anything else you will grow and lower even than blueberry likes. Above about pH 5.5 the plant survives without growing, and above 6.0 the foliage goes pale and the runners stop extending altogether.

That is why a cranberry bed is built rather than dug. Rather than trying to acidify an ordinary garden soil and hold it there, most gardeners excavate and refill with a peat-based mix, or build a raised bed sitting on top of the existing ground. Either way the growing medium is roughly three parts acidic peat or coir to one part coarse sand, with no lime, no mushroom compost, no wood ash and no ordinary all-purpose potting mix anywhere near it.

Building the bed

Cranberry roots are shallow. Almost all of them sit in the top six inches (15 cm), and they never go much deeper than a foot (30 cm). That means you do not need depth so much as a wide, even, consistently damp layer. Eight to twelve inches (20 to 30 cm) of prepared medium is plenty.

1. Pick a site in full sun Cranberries fruit in full sun and hardly at all in shade. Six hours of direct sun is the minimum and eight or more is better. A site that also stays naturally damp, such as the low corner of a garden, saves you a great deal of watering.
2. Excavate or build up Either dig out eight to twelve inches (20 to 30 cm) and line the sides, or lay a timber frame of the same depth on the surface. A three foot by three foot (90 by 90 cm) bed holds three to five plants and is a sensible first size.
3. Line the sides, not the base Pond liner or heavy plastic up the sides stops surrounding alkaline soil and lime-rich water leaching in. Leave the base open, or punch drainage holes in it. A fully sealed bed becomes a stagnant tank.
4. Fill with an acid, moisture-holding mix Three parts acidic peat or coir to one part coarse horticultural sand or fine bark. Wet it thoroughly as you fill, because dry peat sheds water and a bed filled dry stays dry in patches for a whole season.
5. Level the surface A cranberry bed should be flat. Any high ground dries out first and the runners there die back, leaving bare patches that weeds colonise before the vines can close over them.

Where you cannot build a bed, a wide shallow container works. Choose something at least 18 inches (45 cm) across and 10 inches (25 cm) deep, with drainage holes, and expect to water it far more often than a bed.

Setting the plants and spacing

Cranberry is a low creeping evergreen that spreads by runners, so you are not planting individual bushes that stay where you put them. You are seeding a mat that will knit together. Plant closer than feels right and the bed closes over in two seasons instead of four, which matters because bare peat grows weeds faster than it grows cranberries.

Set plants one to two feet (30 to 60 cm) apart in a grid, with the crown level with the surface and the roots spread out flat rather than bunched down a hole. Water in thoroughly, then keep the surface visibly damp for the first month while the roots take. A newly planted cranberry that dries out once often does not recover.

Mulch the bare peat between young plants with a half inch (1 cm) of coarse sand. It suppresses weed seedlings, keeps the surface from crusting, and encourages runners to root as they cross it.

Water quality and the first year

The water you use will decide the pH of the bed over time. Hard tap water carries dissolved calcium and bicarbonate, and every soaking leaves a little behind, so a bed filled at pH 4.5 can drift past 5.5 within a few seasons on tap water alone. Rainwater from a butt is the default, and where you must use tap water, acidify it to around pH 5 for every second application.

Expect very little from the first year. A newly planted cranberry spends its first season making roots and its second making runners, and a real crop is three or four years away. Flowers in the first year are best pinched off, because the plant has better uses for the energy and the fruit would be sparse anyway.

Write down the date you planted, the pH of the fresh mix, and where each plant sits in the bed. Once the mat has closed over you cannot tell the original plants apart, and that record is what explains a patch that never fills in.