Succulents that have been just moved from a shady location to direct sunlight are likely to get a sunburn. The sunburn on a succulent plant shows the worst damage at the top and side facing the sun. Ridges are more likely to burn than valleys along the plant stem. Sometimes, cacti have wide ribs and little spine protection, and then sunburn will occur between the ribs. A light burn will be a whitish discoloring on the exposed areas. More severe burns will become a hard brown scar along with the plant in whatever areas did not get shaded by spines or other parts of the stem.
If you catch a sunburned succulent at the whitish discoloring stage, you can undo the damage by getting it some shade protection. If the plant gets to the brown scaring stage, the damage is permanent, and the plant will have to grow out of it in time. Again, prevention is the best defense against sunburn.
If you purchase a succulent plant sitting inside a store for some time, you will gradually need to move it into full sun. Do this by giving it full sun for a short period each day, and then regularly increase the length of time in the full sun over a couple of weeks. Keep in mind that some species do not ever want full sun all day. The stronger the sun where you live, the more careful you have to be to keep your succulents sunburn-free.
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