What Kills Cacti?

Over Watering

• One of the most common ways these water-hoarding succulents are killed is by over watering. The University of Arizona College of Agriculture and Life Sciences recommends watering in 10- to 14-day intervals in summer. Cacti in clay soils might need less watering; those in sandy soils more. In areas with cold winters, the University of Vermont recommends allowing soil to become totally dry 2 to 3 inches below soil level before watering and avoiding watering on cloudy days. Provide only enough water to moisten the soil around the roots.

  Plant indoor cacti in fast-draining soil mixes, labeled for use with cacti or succulents. The sandy soil mix helps prevent root rot, a plant killer. Pots should also have drainage holes. For outdoor cacti, the University of Arizona CALS recommends adding up to 25 percent of pumice to soil for proper drainage. Amend sandy soils with well-rotted compost or peat moss.

Frost

• Native to the Americas, cacti grow most abundantly in the warm southwestern United States and Mexico. Frost that lasts for a few hours can damage the plant, but the plant will recover. In fact, cacti dehydrate themselves when cold temperatures approach, which prevents their cells from bursting during a freeze. Lengthy frosts or exceptionally cold temperatures can, however, kill cacti. Species have differing temperature requirements, of which cacti owners should be aware. Bring house and container plants indoors. University of Arizona CALS recommends planting cold-sensitive outdoor plants in an area that receives radiated heat at night from a patio or wall. Covering the plant with a cotton sheet can help when frost is mild. Avoid plastic coverings, which only transfer the cold directly to the plant.

Planting Depth

• Cacti have shallow roots. They should be planted at shallow levels in shallow containers. If not planted at proper depths, cacti can die, with roots becoming water-logged and rotted. Outdoor cactus must also be shallowly planted. For example, saguaro, a columnar cacti and one of the most recognizable varieties of cacti, must have all of its green stem tissue above ground.

Disease and Pests

• Although any condition left unchecked might kill a plant if it gets severe enough, very few insects and diseases can seriously harm a healthy cactus. Indoor cacti face attacks by aphids, spider mites and mealy bugs. Light infections may be controlled by removing the pests by hand or washing them off the plant with a stream of water. Insecticide sprays or insecticidal soaps for cacti from garden centers can also be used for control.

  Outdoor cacti may be infected by various types of fungus, which can frequently be prevented by correctly planting and watering cacti. In most cases, removing infected parts or treating the plant with fungicides eliminates the fungus. Bacterial necrosis of saguaro treatment includes removing infected tissues. In advanced cases, the plant has to be removed to prevent spread. Cactus longhorn beetle and cochineal scale insects attack some outdoor cacti varieties. Those can usually be removed by hand or with a strong stream of water.