All gardeners want to ensure their plants get the water they need but many of us worry about whether we have the optimal irrigation schedule. The best frequency at which you should water your garden is fairly uncontroversial, though it does depend on the plants you are growing and what stage they are at so it can be quite complicated to figure out. The time of day at which you should water your garden however, is a hotly contested topic and experienced gardeners often wonder this almost as frequently as beginner gardeners. I advocate for watering in the morning and I think you’ll agree that my reasoning and advice is sound.
How often should I water my garden?
While there is a fair amount of agreement on the frequency at which you should water your garden, it does depend on a few variables.
Has it rained recently?
Firstly, and I hope this goes without saying, you don’t need to water your garden if it has recently rained quite a bit. If you’ve just had a sprinkling of rain that’s a different story but if you’ve had, say, a good 10 mm of rain, your plants shouldn’t need any additional water for a little while.
The first exception to that is if your soil has become very hydrophobic and the rainwater doesn’t penetrate through to plant roots. If this is the case, watering isn’t going to help much anyway and you really need to sort out the hydrophobicity problem before worrying about anything else.
The second exception is if some areas of your garden don’t receive much (or any) rainwater because they are sheltered by a structure or overhanging foliage. If the latter is the case, your plants may stay cool enough and retain enough moisture in their soil (especially if they’re growing in soil that is comprised mostly of clay) that they rarely need watering. If the former is the case, first check that the area isn’t sheltered from the rain but exposed to early morning or late afternoon sun. If it is, it will likely need relatively frequent watering because the sunlight will evaporate the water that is retained in the soil fairly quickly.
What kind of soil do you have?
Soils that contain a lot of clay retain the most amount of water and thus need watering least often. Such soil can pose big problems though if it is allowed to completely dry out so pay extra attention to this.
Soils that are mostly comprised of sand retain the least amount of water and therefore require more frequent watering. If allowed to completely dry out they may become hydrophobic but they are often easier to re-wet.
Soils that are in between these extremes (loams) usually require watering about once a week in moderate weather.
How much organic matter does your soil contain?
Regardless of the type of soil your garden has, the presence of lots of organic matter will allow your soil to retain more moisture and thus it will require less frequent irrigation. In addition, organic matter provides/stores nutrients and helps prevent nutrients from being leached from the soil. It also provides food for beneficial microorganisms that improve the health of your soil. As such, regularly adding organic matter to your soil, in the form of mulch or compost, for instance, is one of the best things you can do for your garden.
Soil that is high in organic matter will be fairly dark brown in colour – ideally the colour of dark chocolate. If your soil is pale, that’s an indication that it is low in organic matter and thus it will likely need more frequent watering.
What kind of weather are you having?
In warmer weather, water evaporates more quickly so more frequent irrigation is needed. Similarly, wind increases evaporation resulting in the need for more frequent irrigation in windy areas. Loam soils might need watering two to three times per week in such weather.
In cooler weather water evaporates slowly and thus less frequent watering is required. In fact, if you water too frequently in cold weather plant roots may rot (a common problem with bulbs) or the soil may become waterlogged, which forces the air out of the soil effectively suffocating roots. As with rot, this can also result in root and, eventually, plant death. Loam soils might only need to be watered once a month in cold weather.
What kind of plants are you growing?
Soil type, weather and plant choice are the three biggest factors when it comes to determining how frequently a garden should be watered.
Established shrubs and trees generally need fairly infrequent irrigation as they have large root systems that can obtain moisture from deep in the ground. Exceptions include citrus fruits (lemon, orange and lime tree for instance) as these have quite shallow root systems despite being trees.
Succulents like to stay fairly dry so don’t need to be watered very often at all.
Seeds generally require the most frequent watering if they are to germinate as they must remain moist for this process to complete. Some seeds, such as those of root vegetables, need to be watered multiple times a day in warm and/windy weather, though there are techniques you can use to reduce the required irrigation frequency (covering moistened seeds with plastic for instance).
Seedlings come in a close second place to seeds when it comes to how frequently they need watering. Because their root systems are so shallow, the top few centimetres of soil must remain moist if they are to survive, let alone thrive.
Some classes of established plants need a lot of water. Examples include water chestnuts and other plants that grow in or at the edge of bodies of water. Cranberries are another good example.
Rainforest plants need fairly frequent irrigation but they also need damp air.
Vegetables also typically require more water than established perennials. In particular, lettuce and other quick growing crops need consistently moist soil otherwise they become bitter.
Fruit producing plants generally require a moderate amount of water. They probably need a similar amount of water as many vegetables except that most fruit bearing plants have deeper root systems so they can grow well with less frequent irrigation provided the same amount o water is provided overall (so each watering session delivers more water than is delivered to vegetables in a given watering session).
Some perennials are particularly drought tolerant and require very infrequent watering once their root systems have become established. Yarrow is a good example.
Because plant water requirements vary so wildly, it is commonly recommended that you group plants with similar water requirements together in the garden as this helps growers provide the correct amount of water for all plants in the garden. I thoroughly agree with this advice.
How do I know if my plants need watering?
So, given all these factors, how can you determine whether your plants need to be watered? Well, despite how complicated the factors are that determine how frequently a plant should be watered, figuring out if a plant actually needs water is fairly straightforward. For most plants, simply push your index finger into the soil near the stem of the plant to such a depth that the soil rises to roughly the height of your second knuckle. When you remove your finger again, if it is coated in soil to your second knuckle, the soil to that depth is moist and so you don’t need to water. If the soil is so dry that it just falls away when you remove your finger then it’s time to water.
For seeds and very tiny seedlings, water whenever the soil on the surface is dry.
Both of these methods work equally well for plants in the ground and those in pots and planters. The only difference is that container plants will always need watering more regularly because there is a smaller volume of soil to hold moisture and they heat up more quickly so lose more water to evaporation.
I recommend you do these soil tests regularly in different parts of your garden throughout the year. The more often you do them, the easier it will be to determine patterns based on the weather and soil in different areas of your garden. After a while, you’ll know whether you need to water without having to test the soil.
How long should I water for? Or, how much water should I provide?
There is a complicated equation you can use to calculate how long you need to water based on how long it takes water to percolate through your soil, the rate at which water flows through your chosen watering tool, the irrigation frequency you choose and the amount of water your plants use. Figuring out some of those variables however, can be challenging. Instead, you may find it easier to use a trial and error approach.
Pick a frequency (once a week in moderate weather is a good place to start) and start by watering for about 30 mins (for an average suburban garden). Or use a 9 L watering can to water a garden bed with an area of 2-3 square metres. Then monitor your plants carefully. If they start to wilt, water them again immediately and either water slightly more frequently for a while or give your plants more water during each session. If your plants cope really well with your chosen parameters, you could try reducing the amount of water or lengthening the time between watering sessions. The idea is to provide enough water that deeper soil is watered and to do this less frequently (only when the soil is nearly dry) as this encourages plants to grow deeper roots, which makes them more resilient and better able to find nutrients (and therefore healthier).
When should I water?
As I indicated earlier in this post, the optimal time of day to water is somewhat controversial. Everyone agrees you shouldn’t water during the hottest part of the day. Doing so is not only inefficient because that is when water will evaporate the quickest, it also risks burning foliage as beads of water can act like magnifying glasses and focus the sun’s rays onto delicate foliage. (Some people believe this is an old wives tale. It’s true that leaves won’t always burn but if it is hot enough and sunny enough it definitely can happen – I have seen it happen, in my own and others’ gardens). The only time you might consider watering when it is hot and still sunny is if plants are wilting and desperately need to be cooled. If you do this, it’s best to hand water so you can focus as much of the water as possible on the soil (this is best practice anyway unless you’re trying to cool foliage) but you may also need to provide them with shade to prevent them from being burned by any stray splashes (shade cloth is good for this).
The controversy starts when you consider whether to water in the evening or morning. As with any good controversy, there are of course pros and cons to both options so let me lay these out for you so I can explain what I think is the optimal solution.
The pros and cons of watering in the evening
- Any water you provide will have the longest exposure to cooler temperatures before it gets warm again the next day and thus plants have more time to use it before it is most susceptible to evaporation. Watering in the evening is most efficient when it comes to water usage.
- This is probably the most convenient option for people who hand water and who are busy away from home all day (not many people want to get up early specifically to water their garden).
- Watering in the evening provides optimal conditions for sails and slugs right when they become most active (when it gets dark) so susceptible plants will be more at risk.
- For the same reason that watering in the evening is most efficient, watering at that time of day also provides the best conditions for harmful microorganisms, particularly fungi, to grow and multiply. This is particularly bad news for susceptible species such as tomatoes and cucurbits.
The pros and cons of watering in the morning
- Watering in the morning means the soil and plants are drier overnight so your garden won’t be as attractive to snails and slugs.
- For the same reason, watering in the morning won’t promote the growth of harmful pathogens as watering in the evening can do.
- Watering in the morning gives plants less time to use the provided water before it is most susceptible to evaporation. This is thus a less efficient time to water.
- This is a less convenient time to water for anyone who is out all day.
There are ways to mitigate each of the cons above to some extent. If you’re out all day, an automatic irrigation system will allow you to water at any time of day so convenience becomes irrelevant. You can reduce the chance of your plants succumbing to a pathogen by avoiding splashing foliage, though only a drip irrigation system will prevent all splashes. You can protect plants from snails and slugs to some extent with copper tape or snail bait but I admit I’ve never found a foolproof way to keep them off their favourite foods. Watering at the optimal frequency and with the best amount of water for your garden will increase the efficiency of your watering sessions.
Given all of this, my preference and recommendation is to water in the morning as the disadvantages of watering in the evening are the most difficult to mitigate and watering in the morning can still be highly efficient if it’s done well.
I hope this helps you determine when to water your garden but if you have questions, please do pop them in the comments section, send me an email or say hi on social media.