Houseplant
Moon Cactus (grafted)
Gymnocalycium mihanovichii (grafted on Hylocereus undatus rootstock) · Cactaceae
Also called: Ruby Ball Cactus, Hibotan, Red Cap Cactus, Chin Cactus, Star Cactus
Moon Cactus (grafted) (Gymnocalycium mihanovichii (grafted on Hylocereus undatus rootstock)) is a low-water houseplant well suited to Tucson and the low desert. It's a slow-growing houseplant.
Moon Cactus (grafted) at a glance
- Water use
- Low (established)
- Sun
- Indoors, bright indirect light. Direct Tucson sun (and outdoor heat) will scorch the colorful, chlorophyll-free top and stress the rootstock, so keep it as a houseplant near a bright window with no harsh midday sun.
- Mature size
- Small: colorful top about 1-2 in across atop a green rootstock stem a few inches tall; a tabletop/windowsill plant.
- Growth rate
- Slow
- Bloom
- Small pale pink to greenish flowers may appear on the scion, but flowering is uncommon and incidental., Spring-summer if it blooms at all (uncommon indoors).
- Cold hardiness
- Tender; keep indoor temperatures above about 50 F (ideal 65-85 F) - both the rootstock and scion are frost-sensitive and intolerant of cold drafts. Grow indoors year-round in Tucson.
- Soil
- Standard fast-draining cactus/succulent potting mix in a pot with a drainage hole.
- Native range
- Gymnocalycium mihanovichii is native to Paraguay and northeastern Argentina; the colorful scion is a chlorophyll-deficient mutant grafted onto a tropical dragon-fruit rootstock (Hylocereus undatus, native to Central America). Not desert-native.
- Best used as
- Indoor/desk/windowsill novelty plant, Colorful gift cactus, Dish-garden accent (short-lived)
- Wildlife
- None - it is an indoor novelty plant.
- Toxicity
- Non-toxic to people and pets. The grafted stock has small spines; handle gently.
How to grow Moon Cactus (grafted) in Tucson & the low desert
Watering
Indoor watering: soak thoroughly, then let the soil dry out completely before watering again (roughly every 1-2 weeks while actively growing, less in winter). Water at the soil, avoid leaving water on the stem, and never let it sit in a saucer - the grafted union rots easily.
Fertilizer & nutrients
Feed lightly with a dilute cactus/succulent fertilizer once a month during spring-summer growth; skip in winter. The scion depends entirely on the rootstock for nourishment.
Pruning & care
Remove any offsets/pups on the green rootstock that divert energy from the colored top. No other pruning; re-grafting the colorful cap onto a fresh rootstock can extend its life.
Notes
Grown strictly as a houseplant in Tucson, never in the landscape. Because the bright red/yellow/orange/pink top lacks chlorophyll, it cannot live on its own and depends on the dragon-fruit rootstock; the grafted pair is typically short-lived (about 1-3 years) as the two parts grow at different rates. It can be re-grafted onto a new rootstock to prolong its life.
Sources: Wikipedia - Gymnocalycium mihanovichii (grafting, chlorophyll-deficient scion); NC State Extension Gardener Plant Toolbox - Gymnocalycium mihanovichii (Moon Cactus); Succulents and Sunshine / mindbodygreen - Moon Cactus care and grafting