Seedling Size and Over-Wintering Temperature

[kkirkland]

Until this year, ALL my cacti overwintered in a bank of South-facing windows with indoor temperatures hovering between 55-70F (18-21C). This year, I have purchased shelves and grow lights, and I plan to overwinter some of my cacti in a cold room at proper dormancy temperatures and humidity. Of course, I will continue to house my Ubelmannia, Matucana, and Melocactus species, epiphytes, as well as my soft succulents, under warmer winter conditions with excellent natural sunlight.

I often see guidance indicating one- to two-year-old seedlings need to be kept warmer and wetter during the winter season. Some of the seedlings in my collection are of the obvious age for these conditions: the small pups I have removed from Coryphantha and Mammillaria, a Sclerocactus wrightii which is about an inch and a half (3.8 cm) wide, a couple of Turbinicarpus schmeidickeanus that are the size of my thumbnail. Other species present more challenges, especially since I have branched into new-to-me species this year. There are the ones clearly more than a year or two old, given their slow growth rate--Ariocarpus in three inch (7.6 cm) pots that are just starting to form their adult stems, the Turbinicarpus pseudopectinatus that's three inches (7.6 cm) tall. But they're still so small! What about the Eriocyses that are all about 3-4 inches (7-10 cm) wide (no flowers this year)? What about the Astrophytums only about 2 inches (5 cm) across (only my larger kikko flowered)? And what about my teeny, tiny Gymno friedrichii variations that actually flowered despite their small size? Can these little guys REALLY handle cool dormancy at their growth stage? Folks with cool dormancy experience: do you base your winter temps on size or age?

[Steve Johnson]

Basic "rule of thumb":

• Overnight lows consistently below the low 50s = dormancy.

• Overnight lows consistently above the low 50s, daytime highs consistently above the low 70s = growing.

Dormant cacti can produce top growth in winter, although from what I've seen in SoCal, the highs need to be in the 80s or higher for this to happen. As far as day and night temps are concerned, a digital outdoor thermometer with a display recording those temps is the best way to keep track of them. Especially important during the transition into and out of dormancy.

My experience with seedlings is limited because I don't have the growing space for a seed-growing operation. However, I do have a few cacti with seedling "volunteers" growing in the same pot with the parent plants. My collection is outdoors, overnight lows in the 40s (upper 30s here and there) during the winter. With a few exceptions, I keep them completely dry in fall and winter. The only thing I'll do with the "volunteers" is spritz a little water on them every 4 weeks or so. Once they're about 3 years old, they're strong enough to go without any water during fall/winter dormancy. Sowing seeds in the same pot with parent cacti may not be the preferred way to go for most growers, so I'll defer to the experts here on how to treat your 1- and 2-year old seedlings.

[kkirkland]

I get that cactus will go dormant when temps are consistently below 50F (10C) at night. Perhaps the better question would be, does a cactus that’s only two inches circumference really need to go dormant? Or does its small size put it at a bigger risk of dying from dormancy temps? Many of my cacti were purchased as small seed-grown todlers. Sadly, they don’t have the benefit of spending a couple of years in the same pot with the mother plant.

Your response was instructive, however. You suggest you overwinter your wee ones at lower temperatures just like the adults.

[Steve Johnson]

Your response was instructive, however. You suggest you overwinter your wee ones at lower temperatures just like the adults.

Correct. Remember that seedlings going through their "tender" stage in the wild undergo the same harsh conditions as their parents, and yet they survive into adulthood. Of course their survival rate isn't that good, but not so much of a problem when you're growing from seed under more controlled conditions. The only drawback I found with seedlings growing in the same pot with the parent cacti -- they grow more slowly. I actually prefer it that way because I like to keep my cactus "families" together unless and until the pots get overcrowded. If I have to "kick the kids out of the house", they'll either go into their own pots or to a local grower.

[keith]

does a cactus that’s only two inches circumference really need to go dormant?

Don't know if they need too but all my seed grown cactus do go dormant and allot smaller than 2"

[kkirkland]

Keith, do you lose any of the small ones during dormancy? That's my biggest worry.

[Steve Johnson]

Keith, do you lose any of the small ones during dormancy? That's my biggest worry.

I think it's more a matter of age, not size -- when seedlings are 3 or 4 years old, they should be able to survive dormancy. But while this may be true in general, it depends on the species. I'm sure Keith can give you more details.

[kkirkland]

That was a big concern, Steve. Many of my new cacti are likely not 3 or 4 yrs old or are just reaching that age, even though some are large enough to be planted in 5 inch pots. Which leads me to believe I need to err on the side of caution and overwinter in a 55-60F (12-16C) environment as well as to continue to lightly water to maintain the ones with more fragile root systems.

It's funny to me that, unlike many folks, I have a room indoors that's unheated and can easily maintain my entire collection at 41-45F (5-7C). And here I am--a month or so before most of my spiky bois hibernate--realizing that my new-to-me species (eriosyce, astrophytum, ariocarpus babies) probably won't appreciate such cold temperatures given their size, age, or stage of root development. It's going to be a serious learning curve this winter!

[keith]

Keith, do you lose any of the small ones during dormancy? That's my biggest worry. "

yes a few they dry out but they are very small