This is my take on how / how much / how often to water Lithops.
The
most important thing is to understand the growth cycle of Lithops.
Lithops grow in a cycle which has identifiable stages; young leaves grow and expand and then flowering occurs. After flowering the mature leaves shrivel while new leaves are formed. In order to facilitate this process
in cultivation, it is advisable to water during certain growth stages
and to withhold water at a certain stage. Fern growers brace yourselves:
Lithops should remain dry (NO watering) for the period starting
after flowering, while the old leaves shrivel and dry out and the new
leaves emerge. This usually takes a number of months and during this
time the plant is able to use reserves in the old leaves to generate new
leaves. In cultivation, it is practical for plants to go through this
stage in winter. The substrate should remain fairly dry during this
period - it's no use having a substrate
which stays wet for 3 months after watering! If that’s the case, the old
leaves probably won’t shrivel and dry as new leaves emerge. More
importantly, the plant might unceremoniously rot and die! Once new
leaves have emerged and the old leaves have shrivelled and dried
completely, watering can start again. In cultivation, this should
usually be around spring or summer.
For Lithops to thrive, the substrate should dry out fairly quickly (a few days at most) after watering. One grower’s plants reside indoors on a windowsill in Germany and grow very well in a 100% pumice substrate. Under such conditions (northern hemisphere, indoors) a substrate should have minimal water retaining capacity. If for example you grow outdoors in a very hot, very bright environment with generally low humidity, like I do in South Africa, you can achieve good results with a substrate which holds more water for longer. This is the mixture I use. Plants grown in a pure pumice substrate or something similar will require more feeding than plants grown in a substrate which contains soil and / or organic matter.
If the plant is an established adult and has completed a year of good growth in a substrate appropriate for the conditions, a drench which wets all the substrate in the pot thouroughly should be in order once old leaves have shrivelled and dried and new leaves have emerged.
Lithops which are plump and firm or swollen have had enough water and only need to be watered again once they start to shrink a bit. Small lines or wrinkles may appear which indicate that the plant is losing water and is starting to shrink. How often you water will depend on your conditions (light, temperature and humidity). I can only speak for my conditions and in the hottest part of summer in South Africa, using the substrate above, I might water once or twice a month. You’ll have to experiment a bit - but wait for the plant to show signs of shrinking before watering.
To summarise, water during spring and summer, only when plants show signs of shrinking and stop watering after flowering, in autumn or winter, so that plants are able to complete the replacement of old leaves with new ones. Resume watering in spring / summer once the old leaves have completely shrivelled and dried out and new leaves have emerged.
If you have a new plant which isn’t well established you may need to be more careful. If you don’t know what conditions it came out of it could be at any stage in the growth cycle. It may have grown for a whole season and want to flower and start replacing its leaves or it may have just replaced its old leaves with new ones. It may be somewhere in between. You'll have to look at it, try to figure out where it is in its growth cycle and then do your best to get it synchronised with your seasons. If you have conditions which are not optimal, I imagine it would be a good idea to get new plants only around spring. If they start to shrink, water, if they're not shrinking, don't. If you get new plants in winter, withhold water as far as possible in order to promote shrivelling of the old leaves and formation of new ones. There is some guessing involved with new plants.
Regarding seeds & seedlings - Seeds need to be regularly watered to germinate. I put some substrate in a pot, cover the top with about 1cm of fine silica sand and sow the seeds onto that. I cover them with a THIN layer of medium silica sand and then water by spraying once a day so that the surface isn't disturbed. Once germination is complete, (usually after about 2 weeks) the watering frequency should be reduced. It will only be necessary to water when the substrate begins to dry out at root level. Stick your finger in to check if you have to. I recommend sowing in spring or early summer. Watch the seedlings carefully and you'll be able to see if they show signs of shrinking. That's a sign that they need water (unless it's a case of damping off and they're disintegrating). In addition, depending on your conditions, you'll see that seedlings can form new pairs of leaves more than just once a year. Under my conditions, they can make 3 - 4 pairs of leaves a year. Once they are full grown, they begin to form only 1 new set of leaves every year.
If you use a sealed container to germinate seeds, watering will only be necessary once after sowing and then again only some time after you've opened the container and the substrate has started to dry out. I don't think it matters much whether you water from the top, from the bottom or by spraying – as long as the tiny plants are not disturbed. Remember that salts tend to build up in the substrate of pot plants which are not flushed from time to time with plain water. It will happen to a greater extent with plants which are watered from the bottom or fertilised often.
For Lithops to thrive, the substrate should dry out fairly quickly (a few days at most) after watering. One grower’s plants reside indoors on a windowsill in Germany and grow very well in a 100% pumice substrate. Under such conditions (northern hemisphere, indoors) a substrate should have minimal water retaining capacity. If for example you grow outdoors in a very hot, very bright environment with generally low humidity, like I do in South Africa, you can achieve good results with a substrate which holds more water for longer. This is the mixture I use. Plants grown in a pure pumice substrate or something similar will require more feeding than plants grown in a substrate which contains soil and / or organic matter.
If the plant is an established adult and has completed a year of good growth in a substrate appropriate for the conditions, a drench which wets all the substrate in the pot thouroughly should be in order once old leaves have shrivelled and dried and new leaves have emerged.
Lithops which are plump and firm or swollen have had enough water and only need to be watered again once they start to shrink a bit. Small lines or wrinkles may appear which indicate that the plant is losing water and is starting to shrink. How often you water will depend on your conditions (light, temperature and humidity). I can only speak for my conditions and in the hottest part of summer in South Africa, using the substrate above, I might water once or twice a month. You’ll have to experiment a bit - but wait for the plant to show signs of shrinking before watering.
To summarise, water during spring and summer, only when plants show signs of shrinking and stop watering after flowering, in autumn or winter, so that plants are able to complete the replacement of old leaves with new ones. Resume watering in spring / summer once the old leaves have completely shrivelled and dried out and new leaves have emerged.
If you have a new plant which isn’t well established you may need to be more careful. If you don’t know what conditions it came out of it could be at any stage in the growth cycle. It may have grown for a whole season and want to flower and start replacing its leaves or it may have just replaced its old leaves with new ones. It may be somewhere in between. You'll have to look at it, try to figure out where it is in its growth cycle and then do your best to get it synchronised with your seasons. If you have conditions which are not optimal, I imagine it would be a good idea to get new plants only around spring. If they start to shrink, water, if they're not shrinking, don't. If you get new plants in winter, withhold water as far as possible in order to promote shrivelling of the old leaves and formation of new ones. There is some guessing involved with new plants.
Regarding seeds & seedlings - Seeds need to be regularly watered to germinate. I put some substrate in a pot, cover the top with about 1cm of fine silica sand and sow the seeds onto that. I cover them with a THIN layer of medium silica sand and then water by spraying once a day so that the surface isn't disturbed. Once germination is complete, (usually after about 2 weeks) the watering frequency should be reduced. It will only be necessary to water when the substrate begins to dry out at root level. Stick your finger in to check if you have to. I recommend sowing in spring or early summer. Watch the seedlings carefully and you'll be able to see if they show signs of shrinking. That's a sign that they need water (unless it's a case of damping off and they're disintegrating). In addition, depending on your conditions, you'll see that seedlings can form new pairs of leaves more than just once a year. Under my conditions, they can make 3 - 4 pairs of leaves a year. Once they are full grown, they begin to form only 1 new set of leaves every year.
If you use a sealed container to germinate seeds, watering will only be necessary once after sowing and then again only some time after you've opened the container and the substrate has started to dry out. I don't think it matters much whether you water from the top, from the bottom or by spraying – as long as the tiny plants are not disturbed. Remember that salts tend to build up in the substrate of pot plants which are not flushed from time to time with plain water. It will happen to a greater extent with plants which are watered from the bottom or fertilised often.
