Thimble Cactus

Thimble Cactus (Mammillaria gracilis subsp. fragilis) is the small, forgiving cactus that suits a windowsill or a rockery pocket. Each stem is a thumb sized cylinder covered in fine white spines, and the plant builds into a low cluster about 15cm tall and 30cm across. Cream white flowers appear on mature clumps, and offsets drop at a touch and root where they land.

Why you'll love it

• Small enough for a sill. A mature clump is only 15cm high and 30cm wide, so it lives happily in a shallow dish or a gap in a rockery.
• It multiplies itself. Detached offsets root without help, which is how one plant becomes a dish garden.
• Not toxic. Unlike many succulents this one is not poisonous to cats and dogs, though the spines are still a reason to keep it out of reach.
• Neglect suits it. Drought tolerant, heat tolerant and happy in poor soil, it asks for less attention than almost anything else.

Where it works best

Pots and containers. A shallow terracotta pot with a drainage hole is ideal, since the wide low shape matches how the plant grows and the clay helps the mix dry out.

Indoors. Give it the brightest windowsill you have. Cacti stretch and lose their compact shape in low light, so this is a bright position plant, not a dark corner one.

Rockeries and balconies. In warm dry gardens it tucks between rocks and gravel where drainage is sharp and reflected heat is high.

Growing Thimble Cactus in Australia

Position and climate. Full sun to very bright light. It suits warm temperate, subtropical, Mediterranean and arid gardens outdoors. It takes light frost only, so in Melbourne, Canberra, Hobart or frosting inland districts keep it in a pot that comes under cover for winter.

Soil. A cactus and succulent mix, or potting mix cut with coarse sand or perlite. Poor soil is fine and rich soil is not helpful. The pot must drain freely.

Water and feed. Soak, drain, then leave it dry until the mix is dry right through: roughly every three weeks in the warm months and once a month or less over winter. A weak cactus fertiliser twice in spring and summer is all it needs.

Pruning. None required. Lift and separate offsets with tongs when the clump outgrows its dish, and let the cut surfaces dry for a day or two before potting them.

What to expect when it arrives

It is supplied as tube stock, so expect a small young plant with one or two stems rather than a full clump. That is normal for a slow growing cactus, and it will build out over a season or two.

Cacti travel very well, though loose offsets may detach in transit and can simply be pressed back into the mix. Leave it dry for a week before the first water. Delivered Australia wide, with delivery from $29.95, and every plant carries our 30 day healthy plant guarantee.

Plant care guides and related reading

• Read our guide to low maintenance cacti for the front garden.
• Browse the full cactus range.
• See other plants safe for pets.
• Compare more indoor plants for a bright windowsill.

Common questions

Is it safe around cats and dogs?

It is not toxic, so a curious nibble will not poison a pet. The fine spines are the real hazard though, so site it out of reach of animals and small children.

How often should I water it?

About every three weeks in the warm months and monthly or less in winter, and only once the mix is dry throughout. Soft or shrivelled stems at the base point to too much water.

Why are the stems stretching?

Not enough light. Move it to the brightest window or outdoors into morning sun, and the new growth will come through compact again.

Will it flower?

Mature clumps produce small cream white flowers, usually in spring. A bright position, a dry winter rest and a snug pot all make flowering more likely.