Cacti and succulents, growing outside, are a common sight in warmer climates, however, they are still very exotic here. But perseverance of gardeners is legendary, as they take pains to grow plants which by default cannot be grown in particular climate. And so it happens, that cacti are moving from cozy window sills into the flowerbeds. Cold hardy cacti become increasingly popular, and the number of amateur growers is on the up, too.
Most cacti and succulents are fairly cold hardy, and easily withstand temperatures down to -20C, or even -30C. But only provided they overwinter completely dry. Continuous winter thaws and cool damp summers are lethal to cacti – low temperature and soggy soil will finish them off in no time. The rule of thumb is simple – if weather is warm, moderate moisture is fine; is weather is cold, cacti should be bone-dry. These are the main requirement for growing succulents outside, and ‘warm’ means 25-30C, rather than 16C.
Cacti should not be planted in a hollow, and planting on a flat surface is not a good idea either (there should be a slight slope from the plant). They grow best in rock gardens, dry walls, on slopes or in large flat containers with a southern exposure. Even in ideal conditions a thick drainage layer is required (about 20-30 cm). The planting mix, made of garden loam and sharp sand at a ration of 1:1, is spread to a thickness of 30 cm. Cacti are planted into small planting holes without disturbance to their root balls. When planting, leave the neck of the cactus exposed, and then fill the gap with sharp sand or gravel to ensure perfect drainage, so as water could drain freely away from the plant to prevent it from rotting.
During summer showers or wet autumn and/or spring cacti are best covered with miniature cloches, made of plastic or glass. There should be a 10-15 cm space between the cover and the plant. You will not need any cloches, if the plants are robust, the summer is dry, and winter is snowy and cold without any thaws. If winter thaws or prolonged showers in summer are forecasted, the cloches might come useful. If plants are less robust, the cloches should stay during the whole winter. In a cold but snowless winter, glass or plastic should be insulated. After a heavy snowfall, remove snow from the cloche to prevent it from caving in. In spring cacti, growing outside, should be protected from scorching sun, just like the evergreen conifers or deciduous plants would be. When subjected to cold, opuntias soften, wrinkle and lay flat on the ground, which makes them very easy to cover for winter even in an advanced age.
Scandinavian or Baltic summers are not sufficiently sunny and warm for cacti and succulents, grown outside. Plants feel best, planted with southern exposure, on a slope or a hillside, next to the house wall, hedge or other vertical surface, which reflects warmth. A choice planting site is next to stones, which absorb heat; the soil around cacti could also be mulches with dark bark chippings.
The outside-grown cacti are plagued with numerous pests and disease, which do not trouble them when grown on a windowsill. One of the major irritants is the perennial weeds, which should be painstakingly removed prior to planting cacti. Mice are also partial to nibbling on cacti.
One of the more simple ways to have a planting of cacti outside is to plunge cacti in their pots in a designated spot. This method is especially convenient when cultivating less robust species of cacti. Come autumn, they can simply be lifted, pot and all, and moved under a roof or somewhere cool. Be careful, when lifting cacti from their place – if the plant enjoyed its summer outside, it may have developed roots, which extend through the drainage hole into the soil. If you plan to transplant the cactus, trim the roots carefully or cut the plastic pot and move the plant into a larger one. In other cases cut the roots and disinfect the wounds. If there has been a recent shower and the soil inside the pot is moist, keep the cacti under the roof in full sun so as the root ball would dry completely. Only then the cactus should be moved to its overwintering place. If you move a damp cactus into a cool location, it will certainly rot.
Grafted cacti can also be grown outside in summer, however, they are best moved inside for winter. According to their cold-hardiness cacti are divided into several groups.
The group of most frost-hardy cacti includes the majority of opuntias (Opuntia darwinii, O. erinacea, O. fragilis, O. howeyi, O. Imbricata, O. littoralis, O. macrorhiza, O. phaeacantha, O. polyacantha, O. rutila) and Maihuenia poeppigii.
The second group includes Austrocactus bertinii, A. hibernus, A. patagonocus, Echinocereus chloranthus, E. engelmannii, E. reichenbachii, E. triglochidiatus viridiflorus, Escobaria missouriensis, E. sneedi, E. vivipara, Mauhuenia valentinii, Opuntia arenaria, O. basilaris, O. clavata, O. nicholii, O. platyacantha, O. violacea, O. whipplei, Pediocactus knowltonii, P. simpsonii, Sclerocactus polyancistrus, S. whipplei.
A number of other succulent plants, which are much more adapted to our climate, can be grown outside – houseleeks, sedums, lewisias, acenas, yuccas and rhadiolas.
© Giedra Bartas, 2010