Moon cactus care guide

The moon cactus is a small, bright ball of red, orange, yellow, or pink sitting on top of a green cactus stem. The color is real, not dye. The colorful top has almost no green, so it cannot make its own food. It only survives because it is grafted onto a green cactus that feeds it. It is a fun, cheap plant that lasts a few years with good care.

How it works

Two cacti are joined together. The colorful top is a mutant form of Gymnocalycium mihanovichii. The green base, called the rootstock, is a fast-growing cactus, often a dragon fruit cactus, Hylocereus. The base makes food for both.

Grafted moon cacti often live a few years. In time, the base outgrows the top, or the join fails. With good care, some last much longer.

Light

Give it bright indirect light. An east window is ideal. A few hours of soft morning sun is fine.

The colorful top burns in hot direct sun because it has no green to protect it. In low light, the green base stretches and leans.

Water

Water when the soil is fully dry. Water until it drains, then empty the saucer. In spring and summer, that is about every two to three weeks. In winter, water once a month or less.

Too much water rots the base. Read how to tell if a houseplant is overwatered or underwatered.

Soil and potting

Use a cactus and succulent mix. A small clay pot with a drainage hole helps the soil dry out.

Most are sold in tiny pots. Move it to a pot one size up the first spring. Use tongs or folded newspaper to hold it. See how to repot a houseplant step by step.

Humidity and temperature

Dry indoor air is fine. Keep it between 18 and 27°C (65 and 80°F), and above 10°C (50°F). The green base is a tropical cactus and does not like cold.

Feeding

Feed two or three times in spring and summer with a cactus fertilizer at half strength. Skip feeding in fall and winter.

Pups and side shoots

The colorful top may grow small pups around its base. You can leave them on. You can also cut them off and graft them onto a new rootstock.

The green base may grow green side shoots. Cut them off with a clean knife. They steal food from the colorful top.

Propagation

The colorful top cannot grow on its own roots. To make a new plant, cut a pup off and graft it onto a fresh green cactus. Cut the top off the new rootstock and press the pup onto the cut, center to center. Hold it in place with rubber bands for a week or two until it joins.

Problems

What you see	Most likely cause
Colorful top fades or shrivels	Sunburn, age, or a failing graft
Soft, mushy base	Rot from too much water
Tan, scabby patches on top	Sunburn
Green base leans and stretches	Not enough light
Green shoots on the base	Normal. Cut them off.
White, cottony spots	Mealybugs

Pests

Mealybugs are the most common pest. They hide between the ribs and spines. Dab them with rubbing alcohol on a cotton swab. Root mealybugs and spider mites can show up too.

Pet safety

Moon cactus is not on the ASPCA list of toxic plants. The ASPCA lists many cacti as non-toxic. The spines can still hurt a curious pet’s mouth or paws.

Varieties

• Red, orange, yellow, and pink moon cacti are all the same species in different colors.
• Green Gymnocalycium mihanovichii is the natural form. It grows on its own roots and blooms with small pink or white flowers.

Some cacti in stores have fake straw flowers glued on top. A moon cactus top is a real, living cactus.