A Guide to Watering Cactus
Almost all cacti have evolved to be adapted to growing in regions with sparse or seasonal rainfall. They are kind of like little water tanks, soaking it up while they can and storing it for times of drought. Some cactus, like the infamous Peyote (Lophophora Williamsii) or common Golden Barrel (Echinocactus Grusonii) have adapted to last up to 4 months without a drop of water. Large, fleshy columnar cactus, like the San Pedro cactus need a little more water than these compact desert species, but correct watering is still a key element to maintaining optimal health of the plant.
What type of water is best for cactus?
Ideally, you want to use water that is pH adjusted, free from contaminants like bacteria or chemicals that may harm our plants, like the chlorine in tap water.
Using collected rainwater is a great option in theory. Rainwater is very low in dissolved minerals and is already pH adjusted in most places. However, many people do not live somewhere with heavy enough rainfall to collect enough rainwater for their plants. And if you do, you will need to think about storage which can be a breeding ground for mould and bacteria. Some people deal with this issue by adding a source of chlorine to their rainwater, but at that point, you may as well skip the middle man and just use tap water. The only real solution to this is if you are able to water your plants with freshly collected rainwater.
In most countries, it is routine for water treatment facilities to add chlorine-containing chemicals to tap water in trace amounts to prevent the growth of dangerous pathogens. These chemicals include free chlorine and sodium hypochlorite. Both of these chemicals can react with organic material in the water lines to generate chemicals known as chloramines, which also aid in sterilising tap water. At the concentrations present in tap water these chemicals will not harm the tissues of a plant, but they will harm the microbiome in your soil. This is obviously undesirable, especially if you're using compost or compost teas or otherwise rely on soil microbes for a healthy plant.
There are several ways people deal with chlorine in water. One common way is to simply let your tap water sit out and allow the chlorine to evaporate off. However, most people don’t realise how ineffective this is. It varies widely based on the shape of your container, but on average it takes a litre of water about a week at room temperature to decant. Even something as small as a 20L bucket could take months to lose all of the chlorine.
In our opinion, the best option is to use a dechlorinating chemical. These chemicals don't actually remove the chlorine, they just render it into a harmless form. Chlorine and hypochlorite and other sterilising chlorine compounds work by oxidising other molecules. By oxidising the molecules that make up the structures of bacteria cells they can effectively destroy the cell. However, these oxidising chlorines don't have the ability to determine what is and isn't a cell, they just oxidise whatever is available to be oxidised. You can take advantage of this by introducing a very easily oxidised chemical that will effectively use up all of the chlorine.
There are two chemicals that we recommend as reducing agents to effectively neutralise the oxidising capacity of chlorine. Sodium thiosulfate and ascorbic acid. Both of these are strong enough reducing agents that they will be preferentially oxidised by chlorine.
Sodium thiosulfate is stable in alkaline solution, so a concentrate can be prepared ahead of time that only requires a few drops to neutralise the chlorine in a gallon of water. When it reacts with chlorine or hypochlorite it generates chloride, a non-oxidizing form of chlorine that is also a bioavailable nutrient, as well as sodium sulphate and elemental sulphur, which proceeds to react with water to form sulphate. Because you only need a very tiny amount, the sodium being introduced to the soil is negligible. Sodium thiosulfate is completely safe to plants, people, and animals. It is commonly used in aquariums to deal with chlorine, as well as in swimming pools to adjust chlorine levels that are too high.
Ascorbic acid, otherwise known as vitamin C, is another good option however it is more expensive and requires more material to neutralise the same amount of chlorine. The resulting products are chloride and dehydroascorbic acid, which is a generally inert chemical.
If using tap water we also highly recommend adjusting the pH to around 5.0-6.0. In most places water comes out of the tap with a pH of around 7.0, but with a cheap hydroponic pH test kit and some pH lowering solutions (both of which you can find easily at hardware shops or online) you can change that. You can buy fancy hydroponic pH-lowering solutions, but using regular white vinegar works just as well. A rate of about 3/4 cup of vinegar per 8L watering can lower 7.0 pH water to around 5.5 pH.
Watering freshly rooted cuttings
If your cactus is already well established with ample roots you can skip this step.
Watering freshly rooted cactus cuttings is a little different from watering well-established cacti. If you give it too much, too soon it will realise it can stop sending out roots to search for water. This can stunt the plant’s root growth and make it weak. It’s kind of like a trust fund kid who’s had everything handed to them, but what we want is a cactus that knows the value of hard work and can put bread on the table one day.
For freshly rooted cuttings we first recommend waiting until the plant shows signs of being thirsty. This will be in the form of obvious shriveling/wilting. Once you’re sure your plant is thirsty try and water the soil in such a way that minimises the likelihood of moisture sitting around the base of the cactus. Two easy ways to do this are watering around the rim of the pot or bottom watering. At this stage you want the roots to sense the water there, to encourage them to grow towards it. Do this until you are confident that the root systems are well established and can handle full soaks.
How often should I water a cactus?
It’s very hard to write instructions on how often to water a cactus, because it is dependent on so many factors: the species, age of the plants, your climate, how much sun it gets, the pot size and type and your soil composition. As an icredibly rough guide, Cactus Culture Australia is based in Brisbane, and we water most of our potted cacti:
Full soak, as often as once a week in Dec, Jan, Feb;
Full soak, every 2 weeks in Nov & Mar
Light watering every 2 weeks in Apr. Sep, Oct
Light watering every 3 weeks in Jun, Jul, Aug
Rather than rely on a fixed watering schedule we highly recommend investing in a soil moisture meter. They are reasonably cheap and are available at most gardening and hardware stores.
With the aid of a moisture meter do the following:
Spring, Summer & Autumn: Soak your cactus whenever it drops to around 15% moisture;
Winter: Just give your cactus a light watering if it drops below 15%. Never go above 40% in winter, and don’t let it drop below 10% at any point in the year.
