Poorly drained soils cause a lot of issues for gardeners. The Plant Company has put together a series of articles to help overcome and remedy drainage issues with practical and proven advice.
Plants generally require good drainage to perform well. A well-drained soil provides air to the roots, which is critical for them to function. When lawns and gardens have poor drainage the result is often poor quality plants, muddy areas, plants dying and looking sick, and generally areas that are not aesthetically pleasing. These areas can become a point of frustration, particularly if solutions are not apparent. The Plant Company has prepared this series of articles to help you grow amazing gardens and lawns through addressing drainage issues.
Soil type and compaction are the two main causes of poor drainage. A soil with a high clay content will tend to hold water, especially if there is nowhere for the water to escape. Thus, a flat site can be quite problematic unless drainage lines, soak pits, or both have been installed. Please see the article, How To Create A Soak Pit For A Garden for how to create these.
The other common cause of poor drainage is soil compaction. Areas that have had a lot of vehicle or foot traffic tend to become hard and impervious to water. If the areas surrounding these spots drain freely, then the fix is simply to cultivate the soil and add compost which will improve the soil structure over the next few months.
A mistake often made by novice gardeners working with poorly drained soils is to add compost. While compost will add organic matter to the soil, it also tends to hold a lot of water. Therefore, adding compost can make the drainage problem worse in the short term. In the longer term it will, however, be beneficial as earthworms feed on the organic matter and create drainage channels. There is a tradeoff that must be considered carefully when adding compost. A better option is coarse plant material such as sawdust or bark if the desire is to increase organic levels.
Clay soils are some of the most challenging to establish lawns and gardens on due to their poor draining ability. There are several common methods of resolving drainage issues on clay soils but all require the ability for water to escape the site.
The first is to remove the clay and replace it with free-draining topsoil. This can be an expensive and arduous effort but is generally fool proof and really the only option if you are trying to grow on pure clay. Remember though, the water still needs to be able to escape.
The second option is to raise the soil level using free-draining topsoil. This is akin to creating a raised garden and causes water to be shed away from desired space. This option is generally accompanied by creating a series of soak pits. Soak pits can be highly effective and our, How To Create A Soak Pit For A Garden explains how to install these.
A third option is to mix coarse sand or pumice with the clay soil. The higher the clay content, the more of the coarse material that will need to be added. This is often done during summer for accessibility reasons. Doing this and raising the soil level at the same time will have enormous benefits for whatever you are wanting to grow.
A mistake often made by novice gardeners working with clay soils is to add compost. While compost will add organic matter to the soil, it also tends to hold a lot of water. Therefore, adding compost can make the drainage problem worse in the short term. In the longer term, however, it will be beneficial as earthworms feed on the organic matter and create natural drainage channels. There is therefore a tradeoff that must be considered carefully when adding compost. A better option is coarse plant material such as sawdust or bark if the goal is to increase levels of organic matter.
Gypsum is often touted as a solution to poor drainage on clay soils. Unfortunately, the benefits of including it do not outweigh the costs and it is best avoided.
The Plant Company considers a poorly drained site as one where water will remain in a hole 24 hours after rain. Please see the article, How To Test If My Soil Is Poorly Drained, for how to test for this.
Rushes such as Baumea and Juncus species, as well as Raupo will grow well on wet soils. These plants are often found naturally in swamps and often in 30 cm or more of water.
Installing a soak pit can be either one large one or a series of smaller ones depending on your situation. For a single soak pit, dig a hole that is at least 700 mm deep and 1 m wide. Essentially, the poorer the drainage, the bigger and deeper the hole needs to be. Make sure the topsoil is set to one side and the poorer draining material is discarded. The Plant Company often uses a mini-excavator to complete this task as the bigger the hole, the better the result will be.
Once the hole is excavated, fill it with pea metal or other rock material up to 200 mm from the top. Avoid using rock that will pack down tightly as this will give a poor result in the long term. Rather, the rock should have minimal fines and be anywhere from 8-50 mm in diameter. Drainage Cloth is laid over top to minimize sediment build up in the rock material. It is not necessary to line the entire hole with drainage cloth if the hole is large. The Plant Company does not recommend lining the base of the hole with drainage cloth as this can impede drainage. The hole is then topped with free-draining topsoil and lawn or other shallow rooting plants grown on top.
If poor drainage is only a minor problem, a series of holes at 3-5 m intervals can give good results. In this instance, dig the holes 700 mm deep and 300 mm wide and fill with material as described earlier in this article.
A hard pan can often be the reason water is not getting away. Therefore, breaking through the pan can be all that is needed for water to escape and drainage issues to be resolved. A pan can be anywhere from 50 cm and more below the surface which is why a mini-excavator is often used.
A simple test that can be undertaken to see if the soil is poorly drained is to dig a hole about 45 cm wide and the same deep. Saturate the area with water such that the hole is full of water and the surrounding soil is thoroughly drenched. Leave the hole for 24-hours. If any water remains in the hole after 24-hours, the soil can be regarded as poorly drained. If all of the water has gone after eight hours, we would describe the drainage as being exceptional.
If all water has drained from the test hole(s) within 24 hours, the options for planting are vast. However, if any water remains in the hole(s), the plant options become more limited unless the drainage issue is resolved. Please see our article, Remedies For A Poorly Drained Soil, for solutions.
The Plant Company considers a poorly drained site as one where water will remain in a hole 24 hours after rain. Please see the article, How To Test If My Soil Is Poorly Drained, for how to test for this.
Perennials such as Canna Lilies, Pratia, Vinca, and Zantedeschia grow okay on wet soils. Whatever can be done to improve drainage will certainly improve the performance of such plants.
There is quite a difference between a poorly drained site and a damp area. The Plant Company considers a poorly drained site as one where water will remain in a hole 24 hours after rain. Please see the article, How To Test If My Soil Is Poorly Drained, for how to test for this. A damp site, on the other hand, is somewhat free draining but the soil is constantly damp for various reasons.
Plants suited to poorly drained sites include:
*Grasses such as Baumea, Juncus, and Raupo; Typha orientalis.
*Perennials such as Canna Lilies, Pratia, Vinca, and Zantedeschia.
*Climbers such as Hedera.
*Trees and shrubs such as Marble Leaf; Carpodetus serratus, Pukatea; Laurelia novae-zelandiae, and Swamp Maire, Willows; Salix, and Poplars.
Plants suited to damp sites include:
*Grasses such as Carex secta, Carex virgata, Carex dispacea, Carex maorica, Cyperus, Baumea, Juncus, Oioi; Apodasmia, and Raupo; Typha orientalis.
*Perennials such as Canna Lilies, Pratia, Vinca, Zantedeschia, Hosta, Astible, Fuchsia procumbens, Colocasia, Ligularias, Arthropodium; Renga Renga Lilies.
*Climbers such as Hedera, Jasmine, Wisteria, and Tecomanthe.
*Trees and shrubs such as Fuchsia excorticata, Coprosmas, Marble Leaf; Carpodetus serratus, Pukatea; Laurelia novae-zelandiae, Swamp Maire; Syzygium maire, Nikau Palm; Rhopalostylis sapida, Puriri; Vitex lucens, Five-finger; Pseudopanax arboreus, Seven-Finger; Schefflera digitata, Cabbage Tree; Cordyline australis, Flax; Phormium, and Macropiper.
A poorly drained site restricts plant options dramatically and you need to do your research before wasting time and money on plants that will fail. The list for plants to avoid for wet sites is long, but the more common ones to avoid are Griselinia, Photinias, Leucadendron, Westringia, Libertia, Hebes, Hydrangeas, Buxus, Camellia, Choisya, Azalea, Rhododendron, and Astelia - to name just a few!
If the plant naturally comes from a drought area or is somewhat fickle, it should be avoided for wet soils.
Fixing poor drainage requires somewhere for the water to go. This article provides a couple of the less expensive options for the home gardener to address problem areas. The first solution is to raise the garden. Adding more topsoil will cause water to be displaced to lower lying areas each time it rains. This can often be sufficient to establish smaller gardens or hedges in problematic areas. Raised vegetable gardens with timber sides is the best example of this method. In those instances, the vegetable garden is usually raised to 600 mm above the ground. However, for a hedge or flower garden the level may only need to be raised 100-150 mm to be sufficient. Once the topsoil is laid, undertake the drainage test as described in the article, How To Test If My Soil Is Poorly Drained.
The second solution is to dig a soak pit or series of soak pits a few metres away from the garden. This is simply a hole filled with stones and topsoil at the top. The notion being that the soak pit will capture water and draw the water away from the garden. The Plant Company recommends digging channels from the garden to the soak pit and laying Novaflo pipe in those channels. The soak pit generally has Filter Cloth around the outside to prevent the pit filling up with sediment.
Some articles will advise that the poorly draining soil is removed and replaced with better draining soil. This will certainly help, but can be fruitless exercise if the water has nowhere to go. Therefore, The Plant Company recommends that before embarking on this expensive and laborious exercise, make sure you plan how the water will escape.
The main signs of a poorly draining soil are water pooling, smelly soil, gluggy soil, or plants continually dying. Water should normally drain from soil within 24 hours of rain stopping, so if water is still present or pooling 24 hours after normal rainfall, drainage is likely to be an issue.
The lack of air in the soil often turns the environment anaerobic and creates the characteristic smell. Often plants will shed their leaves and can end up in a wilted state. Wilting is simply due to the roots not being able to breathe and dying, which funnily enough, reduces water uptake to the leaves. The foliage may be healthy, but because the roots aren’t working, the plant wilts and dies a slow death.
A clay soil is likely to be poor draining over all other soil types. These can be the most problematic for the novice gardener, but there are solutions. Our article, Fixing Poor Drainage On A Clay Soil, provides specialist advice as does, How To Test If My Soil Is Poorly Drained if you are worried that you may have a drainage issue.
The Plant Company considers a poorly drained site as one where water will remain in a hole 24 hours after rain. Please see the article, How To Test If My Soil Is Poorly Drained, for how to test for this.
Trees and shrubs such as Carpodetus serratus, Pukatea, Swamp Maire, Willows, Coprosmas, and Poplars will grow well on poorly drained sites.
Soak pits are often located in the lawn beside a garden as lawn roots tend not to penetrate as deeply as those of shrubs and the like. The soak pit is also aiming to draw water away from the garden or hedge, hence why the location is important. A soak pit can be located in the garden itself with careful management, however. Placing it in a back corner where shallow rooting species will be growing is perfectly okay. The Plant Company often recommends installing them under permeable garden paths or at the lowest points in a garden. Each situation tends to be slightly different, but the key is to ensure water can escape to the lowest point.
There are a host of bad practices for growing plants in pots that have been around for decades. Things such as putting stones in the bottom of pots and using soil are two common examples. The Plant Company has a large-scale plant nursery and through our series of short articles, we aim to pass on our wisdom on plant pot drainage so you can grow better plants and show others the error in their ways.
This is a practice that has been around for generations but is actually detrimental to the plant. The best way to describe this is using the analogy of a sponge. When the sponge is soaked in water and then allowed to drain, there is always a layer (the super saturated layer) at the bottom of the sponge. The last water can’t escape because of the air that surrounds it. When stones are put into a plant pot, the super saturated layer is immediately above the stones. This is not good when the goal was to improve drainage! By removing the stones, the super saturated layer now sits at the bottom of the pot, which therefore increases the volume the roots can explore. Also, stones tend to create large air voids that roots won’t penetrate, which again is not ideal for the plant.
A tall plant pot will always give better drainage and aeration than a shallower one. Imagine you have full bottle of water. You drill a hole at the top and another at the bottom. Water will race out the bottom hole much faster than the top hole. The same principle applies to media in a plant pot. The net result will be a better plant for the same volume of root space due to better drainage and aeration. Of equal note, controlled release fertiliser applied to the top of the pot will be more efficiently used in the tall pot as the nutrients pass through a more concentrated root system before leaving the pot.
Compost is the worst things to pot plants up with. Compost holds a lot of moisture and as such, prevents roots from breathing and growing properly in a potted situation. The result is stunted plants that are often sickly looking. Adding to that, Compost becomes compacted in the pot over time which further exasperates the problem and becomes difficult to rewet when it becomes dry. The Plant Company has seen terrible outcomes with plants potted up using compost. A better option is growing media which has the right drainage and aeration characteristics to ensure your plants thrive. It is also loaded with the correct fertilisers to ensure 12-months of sustained nutrition.
Soil is one of the worst things to pot plants up with. It often becomes compacted in the pot which restricts root growth, and it contains weed seeds. The Plant Company has seen terrible outcomes with plants potted up using soil with plants often exhibiting a long and slow death. A better option is growing media which has the right drainage and aeration characteristics to ensure your plants thrive. It is also loaded with the correct fertilisers to ensure 12-months of sustained nutrition.
Filling a planter box with growing media will give the best results over any other media. Planter boxes greater than 60 cm deep should be filled with free draining topsoil to 60 cm from the top. The rest of the planter should be filled with growing media. Growing media has the right drainage and aeration characteristics to ensure your plants thrive. Do not put stones in the bottom of the planter as this will create other issues as highlighted in the article, Should I Put Stones In The Bottom Of My Plant Pot?. Similarly, do not use soil or compost to the top as reasoned in the respective articles, Should I Use Soil In My Plant Pots? and Should I Use Compost In My Plant Pots?
Planting a wet site that is poorly drained is what we at The Plant Company are great at advising on. We use our specialist plant knowledge and couple that with your goals to come up with amazing results for our customers. We pride ourselves on growing and sourcing the best plants for our customers. Strict quality standards mean that only the best plants will ever be dispatched. We have a wide variety of New Zealand grown plants that absolutely thrive in New Zealand’s climate. Each plant is packed with extreme care, ensuring it arrives to you in the same condition it was in at its nursery. Our impressive google ranking is testament to how well we perform so you can shop with confidence from the best in the industry.
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