Seedlings stopped growing.

[BenCSS]

I have grown a lot of different plants from seed but there is a challenge I encounter with many cactus and succulent seeds. At around the size of a dry pea they stop growing. Eventually some bounce back and start growing again but some never do and eventually die. (currently I have some frailea I started in 2021 the largest is about the size of a dry pea the rest about the size of peppercorn.) So I'm looking for any advice to improve seedling growth. I will provide as much info about my current growing conditions as I can below.

I live in Chicago and so I grow my seeds indoor under grow lights until it is safe to move them outdoors the next summer. I started a variety of different seeds over summer all have stopped growing. I started them in a humidity tray in builders sand washed out with a sieve so its fine but not compact. (In the past I used coconut coir but stopped do to mold and plants damping off.) I left them in the humidity trays until haworthiopsis, psudolithos, &, Whitesloanea crassa seedlings started dying about 5 months later. After that I took them out and cut back on watering to twice a week. PH of water is about 6.3 Temperature average around 72°F/ 22°C. Fertilizer I started with was Bonide Liquid Cactus Plant Food, but I switched to Liqui-Dirt a few weeks ago. (I haven't noticed any change)
Light - Cresmar P1200 LED Grow Light. The lighting varies depending on their position:

120 µmol/m²/s - Psudolithos cubiformis, pseudolithos migiurtinus , Whitesloanea crassa, & Pilosocereus leucocephalus
The Psudolithos & Pilosocereus appear a heathy green color. The Whitesloanea looks kind of yellow/brown looking, not sure if that's normal for it.

70µmol/m²/s - aloe hybrids
They look sun stressed, but I am not very worried about these since I grow a lot of aloe and they always start growing again in Summer.

40 µmol/m²/s - aloe hybrids, & haworthiopsis
These aloe hybrids look healthier.

[Tom in Tucson]

If my anecdotal evidence (FWIW) has any value, it may be you need to cycle your hydration periods for longer intervals. Allowing the soil to dry out somewhat between water applications usually stimulates more robust root development, and consequently further stem development.

I hope other members "chime in" with their own solutions.

[BenCSS]

... it may be you need to cycle your hydration periods for longer intervals. Allowing the soil to dry out somewhat between water applications usually stimulates more robust root development, and consequently further stem development.

Its usually dry within 24 hours. I was letting the Psudolithos & Whitesloanea sit longer without water but it seemed like they were starting to shrivel a little bit.

[MrXeric]

How long are the lights on?

72F is a bit on the low side, try increasing to around 80F. A heat mat can help. On the temperature, it seems unlikely to me that your sand sowing medium dries within 24 hours at 72F. The surface maybe, but I highly doubt the whole pot is dry.

[BenCSS]

@ MrXeric, Lights are on 12 hours, the trays they are in are only about 2-3 inches deep. recently I stuck my finger to the bottom of one the day after watering and it was already dry.

[keith]

one the day after watering and it was already dry. " Maybe that's why they stopped growing ? My seedlings usually slow down or stop as I dry them out I do this to prevent rot . Eventually they will start growing again as I change to a wet then dry cycle like adult plants.

[BenCSS]

@keith, I will try increasing watering on some of them and see if it makes a difference . I'm afraid of overwatering the Psudolithos, & Whitesloanea so I will leave them as is.

[Nino_G]

BenCSS, when seedlings stop growing you can stimulate them by repotting them in the fresh substrate. Take the seedlings out of the existing substrate, wash the roots in tepid water, cut off the excess roots (I usually leave the 2-2,5cm/1" of roots on seedlings of that size), let them dry for 24 hours and then repot them. That should do the trick.

[BenCSS]

@ Nino_G, That seems risky at this point. They are so small I don't want to risk damaging them. Occasionally I've had plants die shortly after repotting them.

[Nino_G]

BenCSS, I wouldn't recommend such thing based on second-hand experience or on something I read at the internet - I do this procedure myself. It is well known fact among growers that relatively frequent transplanting of seedlings enhance growth and I do it 2-3 times during the first year of growth. I currently have Frailea castanea (1 year old) and Frailea grahliana (1,5 year old) that are approximately 1cm in diameter. Using this technique, although it sounds aggressive, rarely results fatalities in seedlings.
Best regads,
Nino

[FredBW]

To me the elephant in the room is 100% washed sand. I see you are using fertilizer. But it is hard to replace nutrients that are in good soil. I believe they are starving.
I have yet to devise my perfect seed starting mix.It varies depending what I'm trying to grow. But I have read in the archives here several places. 1/3 potting soil,1/3 builders sand,and 1/3 perlite. Which is pretty much what I use sifting all through a fine mesh bonsai soil sifter. No set recipe,but 1/3 organic seems to be the norm. Whether potting soil or cactus soil,or compost. But my organic for now is sifted potting soil. I don't use any fertilizer except when I put them outside after it warms up spring/summer.
I do lose a few seedlings shortly after coming up in this mix. But after the first few weeks they all do very well. I've only been doing this a couple years,so I'm no expert. But stop and think about it. There aren't many nutrients in sand. My thoughts are the reason they get no larger than a pea or stop growing is because they have used up all the nutrients that were in the seed.

[keith]

BenCSS, when seedlings stop growing you can stimulate them by repotting them in the fresh substrate"

I do this also and should have mentioned it along with MAYBE more water . I do it with cactus seedlings and Mesembs especially if there is a mold outbreak. short video 'cactus duty' shows me transplanting some pretty small cactus , had to be done . risk losing the whole pot otherwise .

1/3 potting soil,1/3 builders sand,and 1/3 perlite." I use the 1/3 ratio also 1/3 desert soil, 1/3 desert sand and 1/3 pumice ( dry stall ) .

For ME all store bought potting sol causes rot and fungus and too much sand stunts growth after awhile so the re-pot advice is good IMO. I had two pots of conophytum seedlings in mostly sand and after a year just threw the pots away they stopped growing. Went back to desert soil and everything's doing much better just like before and why I switched to compost was just me being lazy the desert is a good 2.5 hours drive . Plus the valley fever coccidioidomycosis but that's another story.

[BenCSS]

@ Nino_G, FredBW, keith
Thank you all for the advice. I have some room for another tray, so I will take some of the seedlings and transplant them into a mix similar to what you suggested. Unfortunately I don't live near the desert so those options are out. But I have perlite and finer soil I sift out out of the mix from my larger plants. I usually use this soil for cuttings and larger seedlings I plan to sell. Young plants seem to grow better in it, but I have never tried it with plants this small. I will take some pictures and see if there is any significant change with them and post back with the results.

[LateBloomer]

I wouldn't change anything with those cacti that stopped growing. IME when they stop growing they usually are adjusting to more mature growth or growing underground. If you are providing water light and nutes leave them alone.

[BenCSS]

Took some suggestions from different forums unfortunately nothing made a noticeable positive difference, except for increasing the brightness of the grow lights a little bit.
Tried repotting some in the recommended ratio 1/3 potting soil,1/3 builders sand, and 1/3 perlite (But I substituted red lava rock because I couldn't find my perlite at the time. These seedling did the worst of the bunch.
Tried increasing watering 3 to 4 days a week on the aloe and Pilosocereus.
Tried increasing the hours of lighting from 12 to 16 hours a day.
Tried changing the fertilizer to Schultz 20:20:20 at 1/4 tsp per gallon
Photos on the left at the beginning of the month/ right at the end of the month.





Still open to any subjections.
I might try watering some daily. Also thinking about trying plant growth regulators.