How to Improve Clay Soil and Poor Garden Drainage
Improving the structure of clay soil
If you can roll a handful of your garden soil into a sticky ball, it is clay.
Clay soils are sticky in winter, but brick-hard in summer, but with care you can improve the structure rapidly.
All clay soils become easier to work with if you add organic material.
Garden compost is best, but soil conditioners such as seaweed, farmyard manure or bagged manure products such as Scott's Organic Dehydrated Manure are also available and will help improve the quality of the soil.
Spread the soil conditioner over the soil, and use a garden fork to turn it in.
This is probably easiest to do in spring autumn after the rains, as clay soil is easier to work with at this time of the year.
Use about a barrow-load of compost or manure to 3 m2 (3 sq.yds) and spread to a 8cm (3") thickness.
Dried seaweed meal also improved the structure of the soil.
Apply 100 - 200 g per m2 (3 - 6 oz per sq. yd).
Follow the instructions carefully when applying proprietary products as they can do more harm than good if applied too thickly.
As someone who has worked with clay soils for many years, on many different locations, I can say that clay soils are improved rapidly when using the technique outlined above.
The soil, within a year or two, becomes much more friable and easy to work with, but it does take many years of repeated applications to reverse the soil structure completely.
Soil constantly encroaches from surrounding areas every time it rains, and dilutes all your efforts.
However, it is well worth making the effort to improve your clay soil structure, as with the passage of time, it becomes much easier to work with.
Raised bed gardens
pH of clay soils
Clay soils tend to be acidic, but not always. In my current garden the soil is not only clay, it is extremely alkaline.
The simple way to find out the pH of your soil, is to buy a soil pH kit.
They are very easy to use and can tell you in just a couple of minutes whether your garden soil is acid or alkaline.
While it is possible to adjust the pH of your soil in order to grow certain plants, you will find this a time-consuming and expensive business.
While adding in compost and organic material is always beneficial, and does tend to help neutralise the soil, you are still going to have trouble growing acid loving plants in alkaline soil, or alkaline loving plants in acid soil, and this is where raised bed gardens come in useful.
Whether you choose to buy a ready built raised bed garden, or have one constructed from wood or brick, you can choose to fill it with a compost of your choice, and grow your plants of choice.
Improve the texture of your heavy clay garden soil, but make a point of growing flowers and vegetables best suited to the natural pH of your soil.
Choosing the right flowering plants for clay soil
If you choose your plants with care, a garden on clay soil can flourish. The selection below all flower at different times of the year, giving your garden a splash of color all year round.
Plants that do well on clay soils
- Flowing currants (Ribes sanguineum)
- Hardy Geraniums (Geranium sanguineum, G. pratense, G. macrorrhizum, G. enderssii)
- Hellebores (Helleborus atrorubens)
- Ivies (Hedera spp)
- Japanese anemomes (cultivars of Anenome hupehensis or A. x hybrida)
- Lungworts (Pulmonaria saccharata)
- Snake's head fritillaries (Fritillaria meleagris)
- Snowdrops (Galanthus nivalis)
- Winter aconites (Eranthis hyemalis)
Plants that will not thrive on clay soils
- Bulbs - avoid the smaller, delicate ones which need light, warm soils.
- Alpine species which are usually grown in rockeries.
- Lilies, except the Madonna lily Liliium candidum and L.pyrenaicum. Lilium regale could also be grown if the soil is cultivated and well-drained.
- Rock roses (helianthemum spp)
- Sea hollies (Eryngium maritimum) and other eryngiums which grow naturally in sandy soils.
- Irises - especially the smaller ones. But Iris foetidissima grows well on clay.
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Clay soil drainage
Clay soils tend to becme waterlogged during wet weather, as the grains tend to stick together and does not let water drain away easily.
Not only do most plants, except bog plants, not like having their feet wet and will quickly rot, it can spoil your own use of the garden.
If you have an area of garden under lawn, you will quickly see damage and moss formations, not to mention the difficulties of traversing waterlogged ground should you dry your clothes outside on the line, for example.
You can help by aerating the ground by making drainage holes with a garden fork or similar tool, but if the problem persists, you may want to consider installing land drainage pipes to remove excess water permanently.
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Land drainage for clay soils
This involves opening up a series of deep trenches across your garden, on areas where you might walk.
You will wish to have a main drainage ditch, about 2 to 3 feet deep in a straight line, in an angle across your garden with a downward slope.
In a herring bone fashion, you will need to dig other trenches the feed into the main trench.
In each trench you lay piping and interconnect them. Buy special drainage plastic tubing with drainage holes across the top to collect the water. Set them down on a bed of stone chippings inside the trench, and angle them so that any water collected will always flow into the main pipe.
Finally you build a soakaway, which is a deep circular hole with sloping sides. You will want your soakaway to be at least 6 feet deep. Position it well away from the house.
Your main drainage pipe should feed directly into the soakaway which you infill with rubble to allow water to collect.
Cover your soakaway with thick plastic sheeting.
In areas of poor rainfall, you may want to build a concrete soakaway to retain water to sustain your gardenin drier times.
French drains for water drainage
French drains are basically just the same as above but without the pipes.
Instead you infill the trenches with rubble and gravel before replace the topsoil.
The idea is that heavy rain will drain away off the surface of your clay soil, which makes walking on it in wet weather more pleasant as it will be drier.
A major difference between the two land drain systems is that when you dig your trenches for pipes, it does not matter if the floor of the trenches do not drop away to the water outlet, as you can adjust the fall with gravel.
With French drains, you must measure exactly the fall of the land to 1:100 as that is the correct level to allow water to drain away.
Clay soil can be a nuisance, but with the addition of organic matter and, where flooding is a serious or annoying problem, the addition of land drainage can make your clay soil garden easy to maintain and enjoyable to use.