Light
Direct sun
Ideal spot
Mammillaria spinosissima 'Red-headed Irishman' · Cactus · pet-safe
Direct sun
Ideal spot
about every 3 weeks
In the growing season
Easy
Slow grower
Safe
Cats & dogs
Red-headed Irishman (Mammillaria spinosissima 'Red-headed Irishman') belongs to the Cactaceae family and is a easy houseplant to look after. This slow grower plant is happiest in direct sun and reaches 5–30 cm indoors.
For watering, the rule is simple: drought-tolerant. In the growing season it needs water roughly every 18 days, dropping to every 40 days in winter. Use the watering calculator below to tune that rhythm to your pot size, light and household humidity.
Good news for pet owners: Red-headed Irishman is listed as non-toxic to cats and dogs by the ASPCA.
Set your pot size, light and humidity — the calculator tunes the watering rhythm to your home.
Light at the spot
Humidity
Season
Water Red-headed Irishman
every 18 days
≈ about every 3 weeks · 1.7× per month
Let the top 2–3 cm dry and finger-check before watering again.
Red-headed Irishman prefers direct sun but will cope with bright, indirect. Harsh midday sun can scorch sensitive leaves, while too little light leads to leggy, sparse growth.
Keep Red-headed Irishman between 15–32 °C and aim for around 30% humidity (15% minimum). Below 5 °C it risks cold damage — keep it away from draughty windows and radiators.
15–32 °C
Not below 5 °C
30%
15% minimum
Drought-tolerant
Soil between waterings
Pot Red-headed Irishman in mineral-heavy cactus mix with pumice or grit. A drainage hole is essential — soggy, airless soil is the single most common way houseplants die.
Non-toxic, but fine spines and glochids can lodge painfully in skin, mouths and paws — handle with tongs.
The best ways to propagate Red-headed Irishman are offsets / pups, stem cuttings and seed. Late spring and summer are ideal, when the plant is growing most actively.
A cactus's spines are highly modified leaves; photosynthesis happens in the green stem, which also stores months of water.
In the growing season Red-headed Irishman needs watering about every 3 weeks (about every 18 days in a 16 cm pot at medium light), and much less in winter — roughly every 40 days. Use the watering calculator above to get the exact interval for your conditions.
Red-headed Irishman thrives in direct sun. A spot near a bright window out of harsh midday sun is ideal; give it a little more light in winter.
Red-headed Irishman is non-toxic to pets. Non-toxic, but fine spines and glochids can lodge painfully in skin, mouths and paws — handle with tongs.
The most common problem with Red-headed Irishman is root rot from overwatering. Check your watering and drainage first — most issues trace back to too much or too little water.
The best ways to propagate Red-headed Irishman are offsets / pups, stem cuttings and seed. Late spring and summer are ideal, when the plant is growing most actively.
Indoors Red-headed Irishman typically reaches 5–30 cm. With the right light and occasional repotting it stays compact and bushy.
Care data last verified on 15 June 2026.
Browse 600+ houseplants with a watering calculator, light needs and pet-safety at a glance.
Browse all houseplants