The problem many of us have is that we collect cacti that are genetically designed the live in these varied climates with a wide range of light, water, and temperature needs and expect them to live and thrive in locales that are not always ideal.
The ideal environment for a cactus is not of course solely dependant on its genetic predispositions. There are several factors that contribute to how well or not well a particular cactus will do under certain conditions. The first factor is how it was raised before you acquired it. Most of the cacti we purchase where probably grown in a greenhouse where conditions are more or less kept within a particular range of light, heat, moisture. Seedlings are kept cooler and in higher humidity conditions with a more limited amount of light than your standard commercial greenhouse. As the plants grow larger they will be moved into a greenhouse where they receive light through opaque plastic and depending on the location of the nursery probably a much hotter and drier climate.
So now you've bought your new little pride and joy from your local nursery, cactus farm, or for some of you mail order nursery. You take it home and immediately place it outside in the hot July sun; probably not a good idea. I have made this mistake a number of times now. On occasion I have traveled to Tucson and purchased cacti at any of the number of world class nurseries in the area. I brought them home to Southern California and put them out on my shelf or deck in direct sunlight. My thinking was originally that these plants were raised in the hot and dry desert of Tucson and should do just fine outside in a slightly cooler climate outside. How wrong I was on a number of occasions. The problem was that even though these plants had been living a greenhouse that in the middle of summer probably reached 115 degrees Fahrenheit they still did not receive any direct sunlight through the opaque plastic of the greenhouse walls. My newly found selections were burning up in the direct sun at home when it was only 90 degrees. Their skins where burning and turning white and soft. They were literally cooking in the direct sunlight. To the lack of shade their chlorophyll was breaking down under the intense sunlight. (Ex. 1, Ex. 2)
I have also had the same experience with cacti that were raised outdoors or in a greenhouse, but lived for a year or more indoors on a windowsill in an air conditioned room. In that year of climate controlled bliss they quickly lost their ability to deal with the stresses of heat and direct sunlight and they too seemed to become susceptible to sun burn when placed outside.
Some cacti have different ways to deal with strong sunlight and heat. For example I have noticed that the Ferocacti of the Southwestern Deserts of the United States develop much more abundant radial spines than the same types of cacti raised in a covered greenhouse. The also will usually have developed better protection and a general hardiness on the south facing side of the plant. Another example of protection from sun would be a cactus increasing the amount of fur it produces. (Ex. 3) Some cacti have fur and or an excess of spines in the area where sensitive new growth occurs which helps shield the tender new growth form the effects of the sun. (Ex. 4) Another style of sun and heat protection would be that of a cactus retreating into the ground to get away from the sun.
A classic example of this would be the Ariocarpus Fissuratus of the Big Bend area in Texas and Mexico whose appearance looks remarkably different in the wild than those grown in a greenhouse. Example 5 shows an Ariocarpus in habitat, example 6 is an Ariocarpus raised in much more favorable conditions. (Ex. 5*, Ex. 6*) Personally I have seen these changes in two particular cacti that I have in my collection. First one is a Mammillaria plumosa that looked fluffier and fuller when I brought it home. (Ex. 7) Then after being placed in the direct sun for several months it became much more withdrawn and the tubercles seems to tighten in on the body making it very hard to see any of the green of the body. (Ex. 8) The second example is a Mammillaria haageana that looked quite normal and vibrant when I bought it. (Ex. 9) After more than a year in the sun it shrank down in size and developed a much denser amount of spines. (Ex. 10)
So the problem is not that some cacti do not have defensive features for extreme heat and sunlight it is that the slow growing nature of cacti makes them susceptible to being placed suddenly in unfamiliar conditions. Your average house plant or tree is able to produce more shade leaves when there is an increase in sun intensity or in if placed in an area with a lack of sunlight shed excess leaves rather easily. A cactus on the other hand is just not able to produce more spines, fur, or toughen its epidermis fast enough to deal with a sudden changes in climate.
In conclusion my advice is to know and pay attention to the conditions your cacti have been in and are in. If you are bringing your cacti out from inside after the winter care should be taken to introduce them to the direct sun gradually. The easiest thing to do would be to use a shade cloth, though I feel in most climates this isn't necessary early on in the spring. It would be better to physically move your plants around alternating them in and out of sun from day to day. For some of us this may be easier than others. In my case I have most all of my cacti on a large shelving unit. (Ex. 11) Certain parts of the shelf receive more or less sun during particular times of day. So what I am able to do is alternately move them around on the shelves depending on what I feel their light requirements are. Also in the middle of summer when the sun is at its most intense I almost always at least in part cover my plants with shade cloth. Cacti aren't able to move or put on sun screen so they may need our help when dealing with the elements. So don't let the sun be an enemy of your cacti.
The ideal environment for a cactus is not of course solely dependant on its genetic predispositions. There are several factors that contribute to how well or not well a particular cactus will do under certain conditions. The first factor is how it was raised before you acquired it. Most of the cacti we purchase where probably grown in a greenhouse where conditions are more or less kept within a particular range of light, heat, moisture. Seedlings are kept cooler and in higher humidity conditions with a more limited amount of light than your standard commercial greenhouse. As the plants grow larger they will be moved into a greenhouse where they receive light through opaque plastic and depending on the location of the nursery probably a much hotter and drier climate.
So now you've bought your new little pride and joy from your local nursery, cactus farm, or for some of you mail order nursery. You take it home and immediately place it outside in the hot July sun; probably not a good idea. I have made this mistake a number of times now. On occasion I have traveled to Tucson and purchased cacti at any of the number of world class nurseries in the area. I brought them home to Southern California and put them out on my shelf or deck in direct sunlight. My thinking was originally that these plants were raised in the hot and dry desert of Tucson and should do just fine outside in a slightly cooler climate outside. How wrong I was on a number of occasions. The problem was that even though these plants had been living a greenhouse that in the middle of summer probably reached 115 degrees Fahrenheit they still did not receive any direct sunlight through the opaque plastic of the greenhouse walls. My newly found selections were burning up in the direct sun at home when it was only 90 degrees. Their skins where burning and turning white and soft. They were literally cooking in the direct sunlight. To the lack of shade their chlorophyll was breaking down under the intense sunlight. (Ex. 1, Ex. 2)
I have also had the same experience with cacti that were raised outdoors or in a greenhouse, but lived for a year or more indoors on a windowsill in an air conditioned room. In that year of climate controlled bliss they quickly lost their ability to deal with the stresses of heat and direct sunlight and they too seemed to become susceptible to sun burn when placed outside.
Some cacti have different ways to deal with strong sunlight and heat. For example I have noticed that the Ferocacti of the Southwestern Deserts of the United States develop much more abundant radial spines than the same types of cacti raised in a covered greenhouse. The also will usually have developed better protection and a general hardiness on the south facing side of the plant. Another example of protection from sun would be a cactus increasing the amount of fur it produces. (Ex. 3) Some cacti have fur and or an excess of spines in the area where sensitive new growth occurs which helps shield the tender new growth form the effects of the sun. (Ex. 4) Another style of sun and heat protection would be that of a cactus retreating into the ground to get away from the sun.
A classic example of this would be the Ariocarpus Fissuratus of the Big Bend area in Texas and Mexico whose appearance looks remarkably different in the wild than those grown in a greenhouse. Example 5 shows an Ariocarpus in habitat, example 6 is an Ariocarpus raised in much more favorable conditions. (Ex. 5*, Ex. 6*) Personally I have seen these changes in two particular cacti that I have in my collection. First one is a Mammillaria plumosa that looked fluffier and fuller when I brought it home. (Ex. 7) Then after being placed in the direct sun for several months it became much more withdrawn and the tubercles seems to tighten in on the body making it very hard to see any of the green of the body. (Ex. 8) The second example is a Mammillaria haageana that looked quite normal and vibrant when I bought it. (Ex. 9) After more than a year in the sun it shrank down in size and developed a much denser amount of spines. (Ex. 10)
So the problem is not that some cacti do not have defensive features for extreme heat and sunlight it is that the slow growing nature of cacti makes them susceptible to being placed suddenly in unfamiliar conditions. Your average house plant or tree is able to produce more shade leaves when there is an increase in sun intensity or in if placed in an area with a lack of sunlight shed excess leaves rather easily. A cactus on the other hand is just not able to produce more spines, fur, or toughen its epidermis fast enough to deal with a sudden changes in climate.
In conclusion my advice is to know and pay attention to the conditions your cacti have been in and are in. If you are bringing your cacti out from inside after the winter care should be taken to introduce them to the direct sun gradually. The easiest thing to do would be to use a shade cloth, though I feel in most climates this isn't necessary early on in the spring. It would be better to physically move your plants around alternating them in and out of sun from day to day. For some of us this may be easier than others. In my case I have most all of my cacti on a large shelving unit. (Ex. 11) Certain parts of the shelf receive more or less sun during particular times of day. So what I am able to do is alternately move them around on the shelves depending on what I feel their light requirements are. Also in the middle of summer when the sun is at its most intense I almost always at least in part cover my plants with shade cloth. Cacti aren't able to move or put on sun screen so they may need our help when dealing with the elements. So don't let the sun be an enemy of your cacti.
From http://cactiguide.com/
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