Cacti have an amazing ability to store water and adapt to new environments. Over-watering combined with poorly drained soil is a recipe for cacti to struggle. The soil should dry almost completely between watering cycles. Check the root zone about 2-3 inches below the surface. If the soil is still damp, wait until it is dry before applying more water. Regularly checking the root zone before watering gives you a good idea of what your plants’ unique watering needs are.
As a general rule, cacti planted in well drained soil can be watered every 10-14 days for one hour (based on using drip irrigation) during the summer months. That amount of time allows the soil to dry thoroughly between watering, yet gives the plant enough supplemental water to survive the heat. As daylight decreases in fall, water should be reduced. As nighttime temperatures drop below 60 degrees, turn off the water completely. Most cacti can survive on natural rainfall alone during winter months.
These watering guidelines apply to most cacti. The one exception is Saguaro cacti. Saguaros should never be watered. If you have a Saguaro that appears to be suffering, consult with your local nursery that specializes in selling cacti.
There is no need to apply fertilizer to cacti planted in the landscape. Cacti planted in pots do occasionally need some fertilizer. In that case, use tomato food (8-20-10) unless you can find specialized cactus fertilizer at a nursery. Apply the fertilizer once a month during the summer months only.
Cacti suffer and often die from three main sources: soil borne pathogens, insects and animals. Pathogens, mostly fungi, affect cacti in many different ways. Normally you will see brown depressions form at the affected area. In some cases, the affected areas can be treated by a fungicide. If the disease has spread to the point where it is highly noticeable, it is better to remove the cactus before the disease spreads to neighboring plants. Proper watering is the key to preventing the spread of soil borne pathogens.
Proper watering also plays a key role in discouraging insects from damaging cacti. Insects are opportunistic and need moisture to survive. Saturating your soil with water is an open invitation to an insect.
Animals, particularly rabbits, find some varieties of cacti tasty and can cause significant damage. The best way to prevent animal damage is to limit access to the plants. Install wire mesh around cacti, especially while they are young. Or simply find a different location to plant your cacti.