How Can You Keep Garden Plants Healthy when Water Supply is a Problem?
Summer has always been a challenging time of year for gardeners to attend to a plant’s water needs. High temperatures and drying winds force plants to use water at a faster rate; water is lost from soil to the atmosphere by evaporation, and unless you live in the tropics there is reduced rainfall.
Climate change is adding to these challenges with longer heatwaves, more intense heat spikes, and less predictable weather patterns. Fighting summer dryness is not so much about a single solution, but about choosing wisely from a range of options.
How do you keep plants hydrated in hot weather?
The simple way to ensure plants are well hydrated is to give them more water. However, water can be expensive. Using more water can also be a problem, particularly in a country where water can sometimes be scarce. Thankfully, there are other ways to make better use of water. Smart water use starts with understanding how water moves through the environment and plants.
Consider some facts:
Plants obtain water from the soil via their roots and store it in their tissues. This water is then used for chemical reactions (metabolism) and is then lost through the leaves.
Water in the soil is available to plants if it is in the root zone, and less useful in soil with no roots.
If water is only near the soil surface, it will evaporate more easily and be lost to the plant.
If the only available water is in the topsoil layer, the root structure of plants tends to be shallow which makes them vulnerable to dehydration.
If water is deeper in the soil profile, roots ten to grow deeper to access the soil. This thicker covering of soil over the roots insulates and prevents them from drying out.
If plants are grown in pots, soil close to the sides and surface or pots can heat up faster and dry out faster on a hot day.
Wind can cause surface layers of soil and leaves to dry out faster in hot weather.
On a hot, windy day some plants can become dehydrated, even if the soil is wet. Water may not move from the soil to leaves fast enough to replace what is being lost.
Some plants are adapted better than others to dealing with water stress.
Some soils hold water better than others but remember too much water can be as bad as too little. Excess water fills pore spaces in the soil and starves plant roots of oxygen.
Checking soil moisture
The most important thing to consider when checking soil moisture is to feel the soil where the roots are, not just the surface you can see. This can be done by inserting the tip of your forefinger into the soil. If you only feel the top 1 cm of soil in a pot or a garden bed, you will be feeling the area that is exposed to heat, where roots might have already been killed off by high temperatures. If you feel 5 cm or more below the surface, you will get a far better indication of how moist the soil is. If it is moist down at this level, there is no need to water. If it is dry, it is time to water.
If you want a more accurate measure, you can use a soil moisture probe that can be pushed into the soil. Those with irrigation systems can also incorporate probes and rain sensors to enable their systems to monitor available water and thereby only turn on watering when it is necessary.
What sort of plants handle water stress better?
Plants have evolved to use different physiological, biochemical, and morphological adaptions to help them cope with drought stress. These adaptations help them to tolerate water stress, avoid it by maintaining higher tissue water content when soil water is reduced, or escape from it such as by completing their lifecycle before the onset of drought.
Some plants have physical modifications that make them better adapted to water stress. When choosing plants, look for those such as the following:
Plants which have thicker, more leathery leaves can be better insulated against heat.
Plants with silver or grey leaves have adapted to reduce water loss with the aide of tiny hairs or waxy coatings.
Succulents that can store and draw on water reserves.
Plants that have adaptations to help them rejuvenate even if top dies back, such as those with lignotubers like eucalypts, or corms, rhizomes, and bulbs.
It should be noted that even drought-tolerant plants will require regular watering for several months after planting until they have established deeper root systems.
Grow Water Efficient Plants
Succulents and cacti: These are known to tolerate water stress well and although many will rot if their roots are kept too wet, they do need watering occasionally. Consider plants like cordylines, agaves, sedums, sempervivum, kalanchoe, senecio, and aloes.
Australian natives: Natives from your own region will be more used to the local climate. Natives known to tolerate periods of dry include many acacias, eremophilas, eucalypts, and correas. Certain species of banksia and grevillea do well in dry conditions.
Mediterranean garden plants: The Mediterranean has hot dry summers so many plants which grow well in that region tolerate summer dryness in the garden. Consider lavender, rosemary, santolina, laurel (bay trees), cistus (rockrose), thyme, and geraniums.
Rock garden plants: Many plants used in rockeries can tolerate summer dry. These include arabis, hypericum, iberis, scaevolas, and phlox.
Other plants to consider include some grasses, some tropical plants like monstera, and plants from countries with hot climates like South Africa. Anything with thicker or more succulent leaves might be more resistant to dryness. Care should be taken not to choose species which could become weedy in your location.
Eleven Water Saving Techniques
Besides choosing water-wise plants, and watering when needed, there are other things to watch out for in the battle against summer dryness.
1. Protection from Wind
Wind, and especially hot and dry wind, increases water loss from plants via transpiration. Plants in pots can be moved to a less windy place or you might use temporary wind barriers such as shade cloth stretched between posts. Permanent plantings might benefit from a windbreak or positioning them next to garden walls.
2. Protect from Direct Sun
Shade cloth can also be used to provide temporary cover from strong sun in summer. You should also consider moving pots under cover.
3. Protect from Reflected Heat
Heat can be reflected from glass, pale brick, pale stone, galvanised steel fencing, or whitewashed walls. It can also be reflected upwards from paving. Plants in areas where sunlight is redirected become hotter than plants located beside a timber fence, hedge, or a bed surrounded by lawn or organic mulch.
Mulches provide a layer that insulates the root zone in the soil from the hot air above. Even stone mulches of rocks or pebbles can do this if laid thickly enough so that they do not transmit too much heat to the soil below. Organic mulches have the added benefit of absorbing and holding moisture, which one would think would always be a good thing; however:
Fresh organic material can have a waxy coating that repels water.
Roots below may grow up into the organic mulch, and as they get closer to the surface, the insulation may decrease.
Over time, organic mulch can decompose and be carried deeper into the soil by earthworms. That increases water holding capacity of deeper soil which helps water conservation. Organic mulches need to be topped up annually to compensate for this decomposition. One of the main mistakes people make with mulching is not using enough. To be effective, a mulch should be several centimetres thick. In some situations, it may even be 15 or 20 cm thick.
Remember a mulch that is dry may be light and full of air. Wood shavings or straw applied at 20 cm thick may end up being 10 cm thick after it has been wettened and settled. If a dry or semi-dry mulch is applied and not thoroughly wet, rain might be soaked up by the mulch and not actually penetrate through to the soil and roots underneath.
5. Wicking Beds/Water Well Pots
Wicking beds are specially constructed raised beds which have a reservoir of water in a compartment underneath the growing media. There are many pots and planters which have similar systems. Usually there is a means of topping up the water in the reservoir via an inlet without having to water the surface of the soil. This way, water is drawn upwards into the soil via capillary action and plant roots grow downwards towards the moisture. Less water is lost via evaporation from the soil and plants can access all the moisture they need.
6. Water Crystals/Wetting Agents
Water crystals hold a reservoir of water in the soil so if the soil becomes too dry plants can access these reserves. Care should be taken not to overuse them since they can cause roots to rot. Wetting agents are liquified additives that serve the same purpose. Whilst once these were environmentally dangerous, modern products do not affect soil microorganisms and aquatic life.
7. Anti-transpirant Sprays
Chemical anti-transpirants can be sprayed onto the surface of leaves to stop or slow down water lost through transpiration. Available from garden centres, these are useful when transplanting. Transpiring water is like human sweating though - imagine how uncomfortable you would feel if you couldn’t sweat on a hot day! These chemicals may help in some situations if used judiciously, but there can be a negative impact of plants overheating.
8. Directed Watering
Different plants have different root systems that spread in different ways. Some plant roots can penetrate the soil as deep as the plant is high, and more than the width of the plant. If the soil can be kept wetter deeper down, then less water is likely to be lost to evaporation from the surface.
Some plants tend to have shallow spreading roots and benefit from more regular watering from drip irrigation. Drip irrigation can also be used for deeper, directed watering. A slow drip of water for a long time will gradually penetrate deeper.
Another directed watering method is to shape the ground surface so as to control where water flows and soaks in across the surface. Swales are a technique that can be used on a slope. A mound can be created along the slope contour to catch water flowing down the hill and make it flow and soak in where it is most needed.
In general, most plants benefit from longer and less frequent watering, allowing the soil time to dry out between watering.
9. Soil Improvement
Heavier soils with more clay and organic matter (less sand), have a greater ability to hold water, but may also have a decreased ability to hold air if they get too wet. If the soil is too heavy it may not absorb water as fast so rain can run off the surface before soaking in.
There is a fine line between a soil being able to absorb, hold onto water, and drain off excess water. Some soil additives are better than others at improving a soil’s wate-holding capacity and maintaining drainage. Cocopeat has become a popular and effective additive that is worth experimenting with; but there are also others.
10. Microbe Management
The soil microbiome is an emerging area of research, showing great potential to improve many aspects of soil. Microbes in the soil are actively involved in facilitating the absorption of water as well as nutrients into plants. A biodiverse planting scheme increases the microbe biodiversity, which can improve the natural capacity of plants to acquire water needed during challenging periods.
11. No-Dig Garden Beds
No-dig garden beds are essentially well-draining compost heaps. They are constructed from a layer of weed-mat (cardboard or newspaper), with a layer of drainage material such as hay on top. This is followed by layers manure, compost, and straw that the plants can then be planted into. This technique optimises water usage