watering in wet weather- your thoughts

[Tina]

I have a friend who grows lots of Euphorbia( president of the Euphorbia society) & he waters his plants when it's bad weather/grey days.
It sort of makes sense as in nature your plants would get watered then, I did try is recently & some of my plants that had been sulking certainly sprouted lots of new growth but with our UK weather you don't know when it's going to be good/warm again.
Any one else heard this or similar ?

[IainS]

but with our UK weather you don't know when it's going to be good/warm again.

Hi Tina: There's always the BBC weather forecast, which is 96% accurate for up to five days ahead.

Yes, it's also my preference whenever the weather cooperates with the watering schedule. During April, I would be more cautious of the week's weather ahead.

I'll send you a PM about a Euphorbia question.

[Phil_SK]

Cacti not Euphorbia, but yes, I aim for dull days - I watered everything this morning. Failing that, I water in the evening. For the final waterings towards the end of the season I'd switch to times when I know it'll warm and sunny for a few days.

[iann]

I aim to water when it is raining. Might just be a superstition, but it seems sensible to me. I do try to time it before a period of decent sunny weather - more important in spring and autumn when the plants risk sitting wet for a long period if the weather doesn't cooperate.

P.S. Rainy weather also means I actually have a good supply of water to use. It quickly runs out in a dry spell and I prefer not to use tapwater.

[Paul in Essex]

I do my watering when I have the time, the inclination and the hose to hand.

[matchat]

I water when I have the time and when the plants need it. If anything I would expect them to benefit from being watered in cool wet weather as they would be able to grow during the daytime without the C.A.M. mechanism kicking in. If your mix is gritty enough and you don't overdo it with the watering can then it shouldn't be an issue. I accidentally overdid it earlier this year with a tray of Mexican miniatures. The weather was overcast and they sat in water for a day and a half at least. None suffered and my Astrophytum asterias hybrids which usually have collapsed sides plumped right up and have stayed plump since then.

Sent from my C6903 using Tapatalk

[ralphrmartin]

Many of my plants now get watered when it rains. That's because they are outside all summer!

[graham]

...If anything I would expect them to benefit from being watered in cool wet weather as they would be able to grow during the daytime without the C.A.M. mechanism kicking in....

I'm doubtful about this, do you have any references to C3-CAM switching in adult cactaceae ?

For a variety of reasons I did have a search some months ago on CAM but saw nothing that suggests an adult cactus has significant C3 photosynthesis (*). I've had another more specific search and found:
http://jxb.oxfordjournals.org/content/e ... rr106.full
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1066319/
http://link.springer.com/article/10.100 ... 993#page-1 - although you don't get to see it all for free

They all suggest that while seedlings and juvenile plants (Opuntia cladodes are the subjects used but it may apply generally) may exhibit C3, and seedlings perhaps exclusively so, as the plant matures CAM switches in. The O. basilaris paper (the third) does make the point that under irrigation there is not a switch from CAM to C3.

I'll admit it's not a very large dataset.

However the seedling results do definitely suggest that avoiding water stress is a useful way to keep up high growth rates by delaying the onset of CAM. And of course that onset may be very dependent on the species.

* The exceptions are Pereskia, perhaps not unsurprisingly and I do wonder about the transient, and some less so, leaves on Opuntia and related genera

graham

[iann]

Yes, people have a habit of making unsupported statements about CAM to support their own habits. There are succulents that have facultative CAM, but no cacti. Constitutive CAM has been observed in Opuntias, with the seed leaves showing C3 metabolism which is the dominant metabolism in the early stages of growth. Adult cacti are obligate CAM plants, with no CO2 uptake during the day whatever the conditions.

Another common "excuse" is that CAM means water is taken up better at night. Again there is no evidence for this Water loss rates are actually (order of magnitude) comparable by day and night (days being hot and dry with stomata closed, nights cool and humid with stomata open), not that this is required for the roots to absorb water.

You can observe interesting CAM behaviour in mesembs. The vigorous shrubby mesembs show a strange type of CAM where the stomata open during the day and close at night. This offers the capacity for rapid growth comparable to C3 metabolism, while allowing the plant to switch almost instantly into a water-saving strong CAM mode without the delays and difficulty of actually changing metabolic pathways. Annual mesembs are often C3 plants that switch to CAM in maturity, typically in response to drought or salinity. Strong CAM "dormancy" occurs much more readily, at lower temperatures or even for obligate periods, in mesembs than in cacti. The few cacti that show strong low temperature or seasonal dormancies are considered "difficult" through a lack of understanding. CAM metablism has been extensively studied in mesembs, with techniques such as carbon isotope ratios being used to identify whether metabolism is taking place during the day or night.

[D^L]

Interesting notes about CAM in mesembs, Ian. Do you have a reference I could persue?

Thanks
David Lambie