Royal Cross/Mammilaria Karwinskiana
Probably the most popular and collected genus of thefamily, plants are fairly easy to grow and many will flower at a small size. Most are globular plants, but some have finger-like stems; they produce rings of small flowers around the crown of the plant, mostly in Spring.
Mammillaria karwinskiana has a light green body and, as it grows larger, produces tufts of white wool from between the short tubercles: old plants can become quite woolly. This species will produce offsets around the base of the plant, but more often the growing point divides and makes two heads. A couple of years later, these two heads will divide to make four and so on, so that the plant makes a nice mounded clump.
The small, dainty, creamy-pink flowers are produced in Spring.
The Royal Cross is an attractive and easy plant to grow, and it will flower when only 5cm (2in) in diameter.
This plant is tolerant of low temperatures (down to 10°C/50°F) if kept dry. Like most plants, it will benefit from good ventilation, but not draughts.
Mammillaria require a bright situation at all times and will grow best in a South-facing window.
Water well during the Summer and then allow theto dry before watering again. During the Autumn and Winter, keep the plant fairly dry, giving only enough water to prevent it from shrivelling badly. This plant will soon rot if kept in wet conditions at low temperatures.
The Royal Cross comes from very arid areas and will benefit from a dry atmosphere.
Feed regularly every 1-2 weeks during the Spring and Summer with a proprietaryfood or a tomato fertilizer at about half strength.
The Royal Cross will do best in a well-drained compost mix containing about one third horticultural grit.
Occasionally a cactus will rot at the base through, over-watering, insect damage or even no apparent cause. If a plant is starved for many years, it loses its vigour and gradually the root system becomes poorer and poorer, which can give rise to all kind of problems.
Should this unhappy event befall a plant, take it out of its pot, cut away all the rotten tissue you can see, leaving only sound tissue exposed. Cut the tissue away little by little to produce a smooth cut. Some Mammillaria have a milky sap and they will exude this when cut, which is quite normal.
Place the cut plant in a dry, bright, warm place with the cut surface exposed to the air until it has formed hard callus. This can take around a week in the Summer, or anything up to a month during the Winter.
When the plant has formed good callus all over the cut end, half fill a pot with cactus compost and then top up with sharp grit (horticultural sand is not suitable). Place the base of the plant in the grit, keep in a bright position and give an occasional light watering. The manner.