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Old February 18, 2023   #1
Greatgardens
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Default Cactus potting mix for seed starting?

I've tried the usual suspects for starting tomato seeds, but I nearly always have issues with the plants being over-watered. The problem seems to be that the mix is dry at the surface, but still wet on the bottom. By the time the plants are about a month old, they are starting to suffer.

I thought I'd try a few plants this season using cactus mix. This is a sandy mix with a little peat and perlite in it. I'll also add a small amount of Tomato Tone dry fertilizer. Has anyone done something like this -- what were your results?

Last edited by Greatgardens; February 18, 2023 at 12:07 PM.
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Old February 18, 2023   #2
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I haven't used cactus mix for seed starting, but I use Promix HP (High Porosity) for all my seedlings. I find it very dry, and I'm obsessive, so I water my seedlings every day, top-watering, using a teaspoon or two. It works really well. However, when I pot the seedlings into Promix (for herbs and veggies) I have to get used to the potting mix needing less water!

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Old February 18, 2023   #3
VirginiaClay
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No personal experience with this, so I googled it and found it's possible to start seedlings even in pure sand. This link is to an old Connecticut Ag Experiment Station study: Sand Study So, your idea probably will work, with the caveat that you may need to fertilize more often (in small quantities), because sand doesn't hold onto fertilizer very well.

I'd be inclined to continue using standard seed starting or potting mixes and figure out the over-watering issue. If you're bottom-watering, maybe switch to top watering? Or just water less often. A dry soil surface doesn't necessarily mean it's time to water, especially if you're bottom-watering. I realize you're an experienced gardener and probably know this already. Sometimes it's hard to resist watering, though!
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Old February 18, 2023   #4
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It's funny though, when I first started gardening (1972), I had zero problems. My first plants from seed were Burpee's Pixie. I built a 4-shelf plant stand with 4 foot shop lights using Gro-Lux WS bulbs, and used Jiffy Mix. One would think that with 50 years practice, I'd certainly not do worse! I guess bad habits creep in.

Last edited by Greatgardens; February 18, 2023 at 03:42 PM.
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Old February 18, 2023   #5
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I went back to square one myself some years ago. Tired of cleaning up the mess of Pro-mix that I spilled, too.
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Old February 26, 2023   #6
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I've also had lots of seedling issues after potting up with the top of the soil drying out while the bottom stayed too wet. The leaves on many seedlings tend to curl up and die. Some varieties seem much more sensitive than others. I haven't found a source for Craig's recommended Metro-Mix 360 in my area or online, so the last couple years I've tried potting up with a 50/50 mix of seed starting mix and cactus, palm, and citrus soil (both Miracle Grow) for most plants and straight cactus, palm, and citrus on some. Plants seemed to do a bit better than with the straight potting soil I'd used previously (which worked fine in 2020 for some reason), but many were still not happy while indoors. All took off quite nicely once it got warm enough to get them outside during the day. I'm wondering if maybe our house is just too cold - ~65 day, 57 night. Thinking I might try some heat mats under the trays that hold the pots to see if that helps. Also can't rule out the LED grow lights, though I had good results back in 2020 with one of the same lights.

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Old February 26, 2023   #7
KarenO
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I use and recommend specifically labelled seedling mix and/ or compressed peat pellets to start seeds.
I then transplant 16oz nursery pots with promix HP.
I choose products without “ moisture control” or fertilizer added as I prefer to control both water and fertilizer myself.
I don’t think I would choose to try a cactus mix, presumably with a lot of sand for tomato or other veggie starts
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Old May 15, 2023   #8
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Quote:
Originally Posted by KarenO View Post
I use and recommend specifically labelled seedling mix and/ or compressed peat pellets to start seeds.
I then transplant 16oz nursery pots with promix HP.
I choose products without “ moisture control” or fertilizer added as I prefer to control both water and fertilizer myself.
I don’t think I would choose to try a cactus mix, presumably with a lot of sand for tomato or other veggie starts
KarenO

This mix has no sand (that I can see) -- peat, perlite, and very small woody fines. I thought it would have sand also. I had really good results this spring with this mix. It drains really well, and with the Tomato Tone that I added, the plants stayed nice and green. This is the best I've done in several years. I've got 3 of my plants in EarthBoxes now, and should get the rest planted this week.

Last edited by Greatgardens; May 15, 2023 at 02:09 PM.
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Old May 15, 2023   #9
KarenO
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Quote:
Originally Posted by Greatgardens View Post
This mix has no sand (that I can see) -- peat, perlite, and very small woody fines. I thought it would have sand also. I had really good results this spring with this mix. It drains really well, and with the Tomato Tone that I added, the plants stayed nice and green. This is the best I've done in several years. I've got 3 of my plants in EarthBoxes now, and should get the rest planted this week.
That’s good news!. Wasn’t it expensive? Here at least I find the specialty houseplant style mixes are a fortune
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Old May 15, 2023   #10
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Yes, about $5 for an 8qt bag. But I only start a small number of plants, so it's not a big expense. I might experiment making my own "high drainage" seed starting mix, but it's awfully easy just to pick up a bag at Menards. Potting mix (like almost everything) has really increased in price this year.

Just out of curiosity, what would be the objection to sand in a seed starting mix? (My homemade cactus mix for my wife is apx. equal volumes of peat, perlite, and sand.) I think if I were trying to use this for seed starting, I would at least double the peat, or maybe even more.
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Old May 15, 2023   #11
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I don’t really have any info on that except it’s heavy . I mostly just use standard seedling mix or peat pellets.
Have you tried the promix HP?
HP stands for high porosity and I like it for potting up
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Old May 16, 2023   #12
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I've thought seriously about it, but the closest supplier is quite a ways from where I live, and it is now really expensive. It was something like $49 for a compressed bale (IIRC). I'd rather buy my meds. :-)
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