Dealing with clay soils and conditioning them for planting

Dealing with clay soils and conditioning them for planting

By Heather Tait – Central Landscapes Planting Expert

Mention that your garden has clay soil and you’ll experience either sympathetic groans for those who have the same, or smug smiles from gardeners with loamy or volcanic soil. Clay is maligned for many reasons: its minuscule particles can form a hard, concrete-like consistency when dry, and a sticky mixture when wet. Clay holds more water than other soils, but often not in a way that allows plant roots to benefit from it.

But it has properties that are beneficial once you control its less than desirable traits. Clay soil contains minerals that are good for plants and, when other products are worked into it, can be an excellent growing media for many plants. Provided the clay has been loosened and ‘worked’ it holds moisture better than many soils, bringing earthworms up to the surface to aerate the compacted soil and aid plants to stay alive in harsh summers. 

Working clay ahead of planting

Autumn is the best season because the soil is warm, moist with the rain and you can walk across the soil without it being muddy and in danger of compacting under your gumboots.

Generally, areas that have not been planted will be a hotchpotch of weeds that colonise easily in clay. They will have well established tough roots that are not easy to remove, because difficult growing conditions attract hardy plants.

It is sometimes easier to spray off the area with glysophate (and a fixing agent to aid penetration) to get a broad coverage to kill off the weeds. With areas that have been left for some time, a second spray will be beneficial.

During this time you can apply gypsum to the surface area. Gypsum is a naturally occurring mineral, Calcium Sulphate, which loosens compacted soil, with no negative impacts on soil pH. (it is also the white product between the paper coating of Gib board). Gypsum is most often available in powdered or granular form and can be spread over the area at the specified rate. After rain, the soil can be dug over with some compost and an optional second application of gypsum. Depending on how far into the dirt you can dig, either leave it to soften a bit longer (more rain required) or begin to dig holes for plants.

Tip:  Potatoes are known for loosening hard soils. If you are a keen vegetable gardener grow a crop of potatoes. After harvest, the soil will be easier to work with, plus there are delicious potatoes to eat.

Planting into clay soils

The general rule of thumb for planting it to dig the hole to the depth of the rootball (pot) and twice the width. It is important that all the rootball goes in the hole as plants with exposed roots tend to stress and die.

Digging hole

Place a handful of gypsum at the base of the hole and add a handful of compost or Garden Mix, before setting the rootball on top. Gypsum will continue to open up the clay for the roots and the growing media offers some extra nutrients. Some landscapers place special fertiliser tablets in planting holes at this time to add additional food, although it is not essential for most plants suited to growing in heavy soil.

Adding gypsum to hole

Before placing the rootball of the plant in the hole, it is important to check the plant roots. If there is a mass of roots and very little potting mix, breaking up the root ball with hands or a knife prior to setting the plant into the hole helps to encourage root growth into the surrounding soil. If you don’t do this, the roots may be so compacted from growing in a confined space that the roots will not extend outwards and grow. Never place a dry plant in the hole – if necessary, soak it first in a bucket of water until the bubbles stop rising.

Placing soil and compost or Garden Mix around the plant is probably the most important part of the process. Digging a hole in clay soil, even if you’ve worked the top area, still leaves the possibility that the hole could become a water reservoir in winter rain. When you replace soil around the plant to firm it in and complete the planting, this should be a blend (layers) of the soil that was originally there and some compost or Garden Mix. The latter can work with the clay to help change the clay soil texture and add a bit of extra nutrient for the plant.

Lastly, apply a layer of mulch even though we’re heading into winter. Why? Among its other characteristics, mulch has large particles which help move rainwater evenly around the soil area, plus it will head off competing weeds while the plant establishes. Never place mulch right up against the stem of the plant though, form a ‘collar’ around it. 

Planting Product List


Fertiliser tablets (if desired)

Compost or Garden Mix


(All products are available from Central Landscapes)

Ongoing annual care

Twice yearly applications in spring and summer of compost or Garden Mix, forked around plants are recommended. If left, clay tends to ‘revert’ to its original structure over a year. A layer of mulch every spring helps retain moisture during dry periods and that, in turn, keeps earthworms near the top of the soil, tunnelling through it and improving permeability.

Tough plants that are great in clay

Many New Zealand plants cope well in tough growing conditions.

These plants establish with ease, after the first year of care:

Trees - lemonwoods and pittosporums, kowhai, ngaio and ake ake

Shrubs - coprosma, camellias, corokias, manukas, some olearias, callistemon, westringea (Australian rosemary), viburnum

Grasses - Carex secta and carex virgata

Groundcovers - grevilleas, coprosmas, ivy

Root rot

Plants that suffer root rot (phytophthora) generally get it when planted in poorly drained clay soils. This can often be seen in rhododendrons and the NZ Griselinia. Reduce the risk by limiting the use of the plants to a few. Use Rootmate, which is a natural beneficial fungus in the hole at planting. It will eat the phytophthora which attracts roots.


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