Cacti in the cold (and high humidity)

[iann]

Standard practice for overwintering cacti in a greenhouse in England is to provide heating which prevents the temperature dropping below a certain point. This is typically 5C-10C (40F-50F) depending on the sensitivity of the collection. The temperature may rise above this level although there will be many days when it does not, depending on whether the greenhouse gets any direct sun in winter. In practice heating a greenhouse means insulating it and keeping it tightly closed, which inevitably means near 100% humidity most of the time. Many growers install a fan to provide some air movement.

I don't consider the constant day and night temperature to be ideal, nor is the very high humidity and the lack of ventilation. My ongoing experiment is to allow most plants to experience natural temperature fluctuations in an uninsulated unheated greenhouse. The cacti are completely dry, some hardy succulents receive a little water. Temperatures inside the greenhouse drop virtually as low as outside, lagging only by a degree or two, which generally means down to -7C (20F) once or twice a winter, and frequently below freezing. There is also a hotbox (coldframe inside the greenhouse) which I aim to keep no lower than about -3C (27F) for slightly less hardy plants. Other plants come inside for the winter. Daytime temperatures are almost always above freezing, generally 5C-10C (40F-50F) and can be up to 20C (70F) even in the dead of winter if the sun really gets it act together on a mild day. The greenhouse is left open many afternoons and evenings when there is little solar heating and no hard freeze expected, this ventilation helps to reduce condensation although humidity still reaches 100% on the majority of nights. Daytime temperatures in February and March are consistently quite warm due to solar heating while nights can still be below freezing. Solar heating on these longer days produces lower relative humidities during the day and also allows more ventilation. Stored solar heat also adds a few degrees to the night temperatures, so by the end of January my plants have probably experienced their worst cold.

I have found this arrangement suits most of the cacti that I grow, and I hope that it will help me with some difficult plants like Pediocactus which I feel are not happy sitting all winter just above freezing. I'm certainly not unique in growing in an unheated greenhouse, but definitely in a minority. Many of my plants have been grown from seed or small seedlings, or obtained from growers with a similar approach. One interesting observation has been an Echinocactus grusonii which originated in the Canary Islands and presumably never saw a frost before it came to me. The older growth marks quite badly in winter, with orange spots developing and then drying to grey-brown scars. Newer growth seems unaffected, presumably it is adapted to the conditions and more resilient.

I am constantly trying new species as I get my hands on disposable plants. I currently have several Copiapoa humilis seedlings which still appear to be alive after a number of freezes. Fingers crossed that they make it through the transition into watering and active growth in spring.

[peterb]

Hi Ian- I had no idea that overnight humidity would be at or near 100%.

I'm amazed at the skill of you and other UK growers who have found a way to grow healthy, flowering cacti under those conditions. It does sound to me that an unheated greenhouse with more ventilation would provide better overall winter conditions than the "resting at constant temperature" regimen. I'm sure the heated greenhouse approach comes from a desire to protect the plants from that mixture of frost and high humidity, quite rare indeed in the habitats of the vast majority of cacti.

I'll be interested to hear how things progress with no heat and more air.

peterb

[lordarutha]

Thanks for that Iann, I need some form of outside storage for my cacti and I was quite concerned about the heating in winter, one thing I would like to ask is I noticed on a post by cactusplantqueen that she has this white sheet stuff to cover her more delicate plants during chances of frost, could that be used on plants that are less cold tolerant?
I am quite lucky living on the coast as it reduces the chance of frost quite a bit but I am still wary of it as my uncle used to collect and one bad frost almost wiped out his whole collection.

[hob]

Standard practice for overwintering cacti in a greenhouse in England is to provide heating which prevents the temperature dropping below a certain point. This is typically 5C-10C (40F-50F) depending on the sensitivity of the collection. The temperature may rise above this level although there will be many days when it does not, depending on whether the greenhouse gets any direct sun in winter. In practice heating a greenhouse means insulating it and keeping it tightly closed, which inevitably means near 100% humidity most of the time. Many growers install a fan to provide some air movement.

that sounds a lot like my setup here, only to cut costs on heating and insulation i move them into the conservatory which is double glazed, south facing and only has 3 outside walls.
i aim at a min temp of 7 deg C 44 F. on sunny days in all but the coldest times i aim to leave the door between there and the house open to stop overheating during the day. my heating is also my fan as i use an electric fan heater to maintain my overnight min temp along with an electric oil filled radiator set on low for nights when the fan heater cannot cope on its own.

My ongoing experiment is to allow most plants to experience natural temperature fluctuations in an uninsulated unheated greenhouse. The cacti are completely dry, some hardy succulents receive a little water. Temperatures inside the greenhouse drop virtually as low as outside, lagging only by a degree or two, which generally means down to -7C (20F) once or twice a winter, and frequently below freezing.

i'm trying something similar on a small scale with about a dozen plants left to fend for themselves in the unheated greenhouse, a few rebutia a mamm a gymno and a large Cleistocactus strausii. so far they all seem to be OK with min temp being recorded as -5 C 22 F.

The greenhouse is left open many afternoons and evenings when there is little solar heating and no hard freeze expected, this ventilation helps to reduce condensation

i don't do that because temps can fall below freezing before i get home from work in the evening.

which inevitably means near 100% humidity most of the time.

even unheated i have seen my greenhouse with so much condensation on the inside of the glass that the plants inside cannot be seen.

One interesting observation has been an Echinocactus grusonii which originated in the Canary Islands and presumably never saw a frost before it came to me.

thats interesting as one of the plants i left out in the greenhouse is a small Echinocactus grusonii.

on another front, it came up on another forum about rebutia not flowering in the tropics, the reason suggested being the lack of a cold winter rest. i have set up an experiment with 2 rebutia seedlings which i put in my refrigerator on Dec 7th to see if they would survive a winter rest in there, so far so good ........conditions in there, total darkness except when opening the door ...... constant temp of 4 deg C and high humidity. i plan to leave them there until mid to late feb.

[hob]

Hi Ian- I had no idea that overnight humidity would be at or near 100%.

this online weather station is about 10 miles from my home, with the time difference between here and the USA you should be able to see conditions here overnight by looking at it in the evening over there.

http://www.wunderground.com/global/stat ... .html?MR=1

[Lewis_cacti]

hob wrote:

it came up on another forum about rebutia not flowering in the tropics, the reason suggested being the lack of a cold winter rest.

yes, i have heard from a grower who recently moved from tropical northern australia to temperate melbourne and he said that it is very rare for some cacti, especially rebutia sp. and some others to flower, due to the lack of cold winter rest.

[iann]

Hi Ian- I had no idea that overnight humidity would be at or near 100%. Shocked

Absolutely. The temperature at night falls to approximately the dewpoint temperature nearly every night of the winter (especially here near the west coast) and often in summer. Inside a greenhouse it will remain at 100% much of the time unless the moist internal air is removed. One thing that this does is to cap the temperature drop because once it reaches the dewpoint it then becomes difficult for the air to cool further. The downside is that copious amounts of dew, sometimes frozen, condenses out on all suitable surfaces including plants. Glass is a very good radiator of heat and so condensation forms readily especially on the roof panels. Greenhouses commonly have small gutters on the inside to collect condensation which runs down the glass. Condensation may also drip onto plants. So ventilation is essential, but must be balanced against any advantage of solar heating during the day which can reduce relative humidity by increasing the temperature.

Anyone planning to expose their plants to frost should remember that I don't grow Melocactus. Discocactus, Euphorbias, or epiphytic cacti. Nor do I have many Matucanas, or those lowland Brazilian weeds Just for interest I do have Neoporterias, Copiapoas, Lithops, all of which can take some frost for brief periods. I have also had a Madagascan Euphorbia, Matucana madisoniorum, Eriosyce aurata, and numerous other "non-hardy" plants down to freezing between otherwise warm (30C/86F) days. I do overwinter a number of species not normally considered hardy; Matucana polzii (and some other Matucanas), Lithops hookeri (and some others), and Sulcorebutia rauschii are all hardy to at least -7C (20F) when dry. There some other plants like many Notocactus which can survive very cold conditions but regularly lose their roots if kept completely dry and cool for many months over winter, so at the moment I just keep these inside the house and water them a couple of times over winter. I also know of growers who have tried to switch from a heated approach to a non-heated approach and suffered losses of plants which I can overwinter without trouble. This may be due to the plants never having experienced freezing before or it may be due to unsuitable arrangements for controlling condensation and humidity.

Placing a covering over plants can help them in two ways. First is by trapping heat to prevent warm air convecting away. More important is the reduction of radiative cooling by providing a radiant barrier between the plants and the sky. You should use something like paper or fleece to prevent condensation sitting on the plants. Newspaper is very good and available in handy sheets which can be arranged to cover plants as necessary.

[iann]

Update on humidity: my local weather station on the internet is showing humidity 100%, temperature 5C (41F). Its is nearly 3pm so this is about as good as it will get today.

My greenhouse has reached 7C (45F) from the small amount of heating we get even on cloudy days and this has encouraged the relative humidity in there down to 98%! There is still heavy condensation on the roof panels and a little on the side panels. I'll open it up soon and see if the condensation will clear.

You can probably imagine that it isn't necessarily the cold that kills most plants here in winter

[peterb]

Yes, the moisture is much more dangerous I would imagine. This helps me understand why the "minimum temperature" guidelines provided in some books by authors from temperate climates are as high as 50F/10C, for plants otherwise safe in habitat to 20F/-7C or lower if dry. But if air circulation is improved and the plants don't grow moss or get coated with a thin layer of ice and then get watered by the roof every day, the temps should be able to go lower. Obviously, the higher recommended temps are also an effort to dry things off more quickly after nights with 100% humidity and dripping dew. I wonder if cold, dry storage such as in a refrigerator (where humidity is generally very low) wouldn't completely solve the problem. In a perhaps completely impractical way.

The conditions you describe also help me understand why some genera have traditionally been characterized as "difficult in cultivation" when the plants seem to be fairly easy under more dry conditions.

On the other hand, we've been reaching dew point conditions every night for the past 6 weeks or so, and in the Sonoran anyway this ephemeral but repeated thin layer of water seems an additional factor in the "winter growing season" that I'm still learning about.

peterb

[hob]

The conditions you describe also help me understand why some genera have traditionally been characterized as "difficult in cultivation" when the plants seem to be fairly easy under more dry conditions.

books written by uk authors like John Pilbeam will reflect conditions in the uk

current outside conditions here are

Temperature: 33.2 °F / 0.7 °C
Humidity: 83%
Dew Point: 28 °F / -2 °C
Wind: Calm

[iann]

Humidity: 83%

You lucky people from the desert south east

[peterb]

For Tempe AZ this morning:

Temp: 30F/-1C
Humidity: 36%
Dew point: 11F/-11C

it's a dry cold....

peterb

[CoronaCactus]

Corona, CA. conditions as of 7:53 am PST

Temp: 53°F / 11°C
Barometer: 30.08 in and steady
Humidity: 11%
Dewpoint: 0°F / -17°C
Wind: E 21 mph (with gusts up to 60mph!!!)

It's a bit dry here too

[daiv]

Long Prairie, MN conditions as of 2:30 PM CST

Temp: -6°F / -21°C (yes as in negative)
Humidity: 59%
Dewpoint: -15°F / -26°C
Wind: WNW 16 mph

"Feels Like" temp = -27°F / -33°C

[daiv]

By the way Ian, I a have been thinking about your humidity/heating comments. I know a heater such as an oil-filled radiator won't decrease moisture in the air, but how about an electric heating element or propane heater? I would think those would dry the air as they operated, no?