Moon Cactus

Gymnocalycium mihanovichii (grafted onto a Hylocereus rootstock)

Succulents & CactiEasySlow (the rootstock outgrows the top)🐾 pet-safe

Good to know: The colored top is a mutant that can't make its own food, so it only exists grafted. It may flower in spring with pink blooms. The colors are natural, not dyed, unlike the glued-on "flowers" some cacti are sold with.

Through the year 🗓️

🍂 Fall now

Taper watering and stop feeding as the days shorten.

❄️ Winter

Keep it bright and above 50 °F, and water only every 4–6 weeks.

🌱 Spring

Growth starts: resume regular soak-and-dry watering and monthly feeding.

☀️ Summer

Water when completely dry, and shade the colored top from midday sun.

The details

Light & placement

Light: Bright indirect with gentle morning sun; the colored top has no chlorophyll and scorches in harsh direct sun
Ideal PPFD: 150–350 µmol·m⁻²·s⁻¹
Grow-light distance: 14–20 in below LED

Water & feed

Watering method: Soak-and-dry: water thoroughly when the soil is completely dry, then let it drain. Much less in winter.
Fertilizer: Cactus feed at ½ strength monthly in spring–summer; none in winter.

Home & habit

Soil: Gritty cactus mix with extra perlite or pumice; pH 6.0–7.0
Humidity: Low (20–40%)
Temperature: 65–85 °F; keep above 50 °F
Mature size: 4–8 in in all
Repotting: Every 2–3 yrs in a snug pot, handling it by the rootstock
Propagation: Pups from the colored top can be grafted onto a fresh green rootstock. They can't root on their own.

When something looks wrong

Overwatering signs: Soft, discolored rootstock; the top loosens or rots at the graft
Underwatering signs: The green stem wrinkles or shrinks and the top looks deflated
Common pests: Mealybugs, spider mites, scale

Care notes: It's two cacti joined together: a red, orange, yellow or pink Gymnocalycium with no chlorophyll, kept alive by a green rootstock that does all the photosynthesis. Most live 2–5 years before the two parts outgrow each other, and regrafting the top onto fresh stock can extend that. Rub off any shoots from the green rootstock so it feeds the top.

Common problems 🩺

The usual suspects when Moon Cactus looks unhappy. Most houseplant trouble is watering or light, not disease — see all plant problems if you are not sure which plant is the culprit.

Mushy base, or the colored top coming loose

Likely cause
Overwatering, which rots the green rootstock.

What to do
Let it dry fully between waterings. If the rootstock has rotted, the top can be regrafted onto a fresh one.

Colored top fading or bleaching

Likely cause
Harsh direct sun burning a top that has no chlorophyll to protect it.

What to do
Move it to bright indirect light with only gentle morning sun.

White cottony tufts between the spines

Likely cause
Mealybugs.

What to do
Dab them off with a cotton bud dipped in rubbing alcohol, then check weekly until none come back.

More like this 🌿

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Aeonium

African Milk Tree

Bunny Ears Cactus

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