Preparing a new garden bed

Almost every disappointing planting can be traced back to the bed rather than the plants. Ground that was never opened up, drainage that was never tested, a compacted layer left in place under 100 mm of imported topsoil: these are invisible on the day and decisive three years later. Bed preparation is unglamorous, and it is where the outcome is actually determined.

When to do this work

Prepare beds in late summer or early autumn, so they are ready for the main autumn planting window. In the tropics, prepare in the late wet so the bed is ready for the early dry. Working the soil while it is just moist is far easier and far better for structure than working it dry or saturated.

Allow time between preparation and planting. A bed that has been watered, settled and levelled is a better bed than one planted the same afternoon, because the settling reveals problems while they are still easy to fix.

What you need

A spade, a garden fork, a rake, a wheelbarrow, a hose, and gloves. A mattock is useful in hard ground. Nothing on this list is specialised, and none of the steps below require machinery for a domestic-scale bed.

The nine steps

1. Mark the bed out and live with it for a week

   Lay a hose or a length of rope on the ground in the shape you are considering and leave it there. Look at it from the windows you actually use, from the front door, and from the far end of the garden. Beds almost always want to be larger and simpler in shape than they first appear on the ground.

   Make the bed at least 900 mm deep front to back if it will hold shrubs, and 1.2 m or more if you want layered planting. Anything narrower than about 600 mm cannot hold a shrub at its mature width and will need constant cutting back.

2. Check what is underneath before you dig

   Locate services before breaking ground: water, gas, electricity, irrigation and stormwater lines are frequently shallower than expected, particularly around older houses and near meter boxes. Contact the national referral service for information on underground assets before any digging that goes deeper than a spade’s width.

   Look up as well. Note where existing tree canopies will cast shade in winter, when the sun is low, rather than in summer when you are standing there planning.

3. Test drainage and soil type

   Dig a hole 300 mm deep and 300 mm across somewhere in the middle of the marked area. Fill it with water, let it drain away completely, then fill it again and time the second draining. Two to four hours is good. More than four hours means the bed needs raising. More than twelve hours means it must be raised.

   While the hole is open, work a small handful of moist soil into a ribbon between thumb and forefinger. A gritty mix that will not hold together is sand; a smooth ribbon over 50 mm long is clay. This determines both what you add and what you can plant.

4. Clear the existing vegetation

   Remove grass and weeds physically. For a small bed, cut the turf into strips with a spade and lift it; the strips can be stacked upside down in a corner to break down into useful loam over a year. For a larger area, sheet mulching works: lay overlapping plain cardboard directly on mown grass, wet it thoroughly, and cover with 100 mm of mulch. Leave it for two to three months before planting through it.

   Dig out perennial weeds by the root rather than cutting them off at ground level. Anything left behind will come back through the new bed, and it is far easier to remove now than later.

5. Break up compaction

   Push a garden fork in to its full depth every 200 mm across the whole bed and lever it gently back and forth to crack the profile. Do not turn the soil over. The aim is to open the ground so roots and water can move through it, not to invert the layers.

   Do this when the soil is just moist. Forking dry ground is exhausting and ineffective, and working wet clay smears it into a sealed layer that takes years to recover.

6. Decide whether to raise the bed

   If the drainage test took more than four hours, build the bed above the existing grade rather than digging into it. Two hundred to 300 mm of additional soil above the surrounding level puts the root zone above the saturated layer, and it works regardless of what the subsoil is doing.

   Do not dig a deeper hole and backfill it with better soil in heavy ground. It becomes a sump: water flows in from the surrounding clay and has nowhere to go, and the plant sits in a bucket.

7. Add organic matter

   Spread 50 to 100 mm of compost or well-aged manure over the surface and work it into the top 150 mm. In sandy soil this is what will hold water and nutrients. In clay it separates the fine particles and creates the pore space clay lacks.

   Use manure that has genuinely aged — at least six weeks broken down. Fresh manure worked in immediately before planting burns young roots and causes forked roots in carrots. Avoid adding sand to clay in an attempt to lighten it; in the wrong proportions it sets harder than the clay did.

8. Shape and level

   Rake the bed to a consistent surface with a slight fall away from any building. Break up clods larger than about 30 mm. Remove stones, roots and construction debris as you go.

   Leave the finished surface 50 mm below the level of any adjoining path or lawn edge, so that mulch can be applied later without spilling over. This detail saves a great deal of sweeping.

9. Water, settle, then plant and mulch

   Water the whole bed thoroughly and leave it for a few days. This settles the soil and reveals any low spots, which are far easier to correct now than after planting. Top up hollows and rake level again.

   Plant, water each plant in with a full bucket to settle soil around the roots, then mulch to a settled 50 to 75 mm, keeping the mulch 50 to 100 mm clear of every stem and trunk.

Common mistakes

Importing topsoil over compacted ground. The most common shortcut on new estates. The new soil looks excellent and the plants root only within it, because they cannot penetrate the pan beneath. The bed then behaves like a shallow container, drying out fast and waterlogging in winter.

Digging a deep hole in clay and filling it with good soil. Discussed above and worth repeating, because it is advice still widely given. It creates a sump.

Adding sand to clay. Unless the proportion is very high, sand plus clay makes something closer to concrete than to loam. Organic matter is what improves clay.

Making beds too narrow. A 500 mm border cannot hold anything that will not need cutting back twice a year forever. Wider beds are less work, not more.

Planting the day the bed is finished. Water it, let it settle, correct the hollows, then plant.

What comes next

Once the bed is built, choose plants for the soil you have actually got rather than the soil you hoped for. If the drainage test ran long and you did not raise the bed, plant from the wet-tolerant list. Use the plant finder to filter by soil type and water need, and read soil and mulch for the ongoing maintenance the bed will need.