Seed sowing woes Solved

[rreynolds]

I have been growing cacti from seed for eight years. I immediately adopted the baggie method because the idea of a (mostly) self-sustaining micro habitat appealed to me. I didn't have to check on everything daily--I could leave them for weeks if necessary and they would be fine. I live in a humid climate (Austin, TX, zone 8) so open sowing presents problems of its own with algae, mold, etc. Plus, I cannot monitor everything on a daily basis since I tend to travel frequently.
Recently the baggie method has been a source of frustration. Despite my best efforts I am getting algae and mold blooms as soon as one month after sowing. Some of the molds are very aggressive and kill seedlings unlike algae.
Here's what I've been doing:
Put my soil-filled pots in a glass baking dish into which enough distilled water is added to saturate the soil completely. (I use distilled water because our tap water has a pH around 9.8.)
Microwave everything until soil reaches 212°F, then microwave for a few minutes longer. (I have to be careful with this because I have already destroyed one magnetron.)
Let everything cool to room temp with a cover over the baking dish (which itself has been wiped down with 91% alcohol).
Clean my hands with hand sanitizer before touching anything which may come in contact with seeds and pots.
Dip all spoons, pencils, tweezers, etc. in 91% alcohol before using.
Briefly dip all plastic labels in 91% alcohol (10-15 sec) and let dry before inserting in pots.
Sow seeds one pot at a time to reduce exposure to air.
Spray seeds with distilled water and let soak (covered) for 15 minutes so the firsst thing they soak up is pure water.
Spray seeds with Physan 20 mixed at recommended dilution (2.5 ml per 32 oz. distilled water). Note: I know some people consider Physan a germination inhibitor but I have run side by side tests and not found this to be true.
After Physan spray I put 2x2 pots in new individual poly zip bags into which I have sprayed Physan 20 and then close them.
I used to also spray seeds with Chinosol in my early days of seed-sowing and rarely had algae or mold infections for the first year. I recently added Chinosol back into the routine but am still getting soil infections.
My soil mix is typically a blend of small pumice, calcined clay, scoria, screened coarse sand, vermiculite, and 20% or less of coco coir. I don’t have access to good quality sandy loam. I always test my soil mixes for good drainage.
Any thoughts?

[Astro]

I usually fill 2.5" square pots with soil mix and let them soak up tap water by standing them in a water filled tray. Then they go into zip lock bags (2 will fit in one bag), I add some extra water to the bag (maybe 5-10cc) and then each bag goes into the microwave for 2.5 minutes. I keep the bag open so steam doesn't rip everything apart. Then let things cool down and sow the seeds (take the pots out one at a time). Then zip the bag up (can be a bit problematic sometimes as the plastic can warp). I haven't had any issues with algae or fungus even for cacti that stay in the closed baggies for a few months.

[Terry S.]

I don't use the bag method because I am retired and at home most of the time (all of the time now!) and can use easier conventional techniques. Analysing what you have done, I can see two weaknesses. First, you do not say much about sterilising your seeds just that you spray them with a product that is unfamiliar to me; soaking in peroxide or hypochlorite are the usual agents. Second, do you keep exposure to air of sterilised materials to a minimum? A professional propagator would use a laminar flow cabinet for all such manipulations, so can you think of some way to minimise exposure and thus avoid all of the spores floating around us. Quite pertinent thoughts really in these days of avoiding exposure to COVID-19 particles. Perhaps wearing a face mask would reduce airflow around materials while you are manipulating them?

Reading something on raising orchids from seed should give some ideas on how to keep things sterile.

[Herts Mike]

That all sounds incredibly complicated and for little return. I've been growing from seed for ever with no problems at all.

Clean pots, compost and vermiculite 50/50 (John Innes which I appreciate you don't have in USA). New plastic bags.

But then having sown the seed the first watering is by standing the pots in boiling water until it has soaked up. Leave to drain then into plastic bags and away you go. I am convinced the boiling water stops a lot of problems.

[MatDz]

But then having sown the seed the first watering is by standing the pots in boiling water until it has soaked up.

Is this with plastic pots? I have some 2" BEF pots I bought 2nd hand and, as this method sounds appealing, I would rather not melt them when testing!

[Chris43]

Wow, that's some process! Not sure what I'd suggest to change.

Are your pots plastic or earthenware? If the latter could there be residual spores deep in them which aren't killed in the microwave? I leave my sown pots to soak for a good length of time, so the seeds are fully wetted with the boiled chinosol infused water. Could there be something there?

My method is similar but simpler. I wet my soil (50-50 JI and volcanic grit)with tap water, in to the microwave until steam is coming off, let cool, put into well washed 2" sq plastic pots. Sow seeds and bottom water with weak Chinosol solution using boiled water. then into new bags and into propagator. I have each pot numbered with Brother tape label stuck on the outside of the pot so no need for labels in the soil at this stage. I've been doing this for quite some years, and mostly haven't had any problems, just occasionally a bit of algae a couple of years ago, which I put down to uneven soil sterilisation, but mostly OK. Nothing since then.

[Mal H]

To focus on your method, when you spray your seeds with distilled water - is that sterile?

[daydreamer]

What do you put in your compost mix to provide nutrition for the seedlings? What light source and what intensity? I wonder if your conditions are favouring the algae and the moulds rather than the seedlings. I use similar methods to you, but without the precautions, except for using rainwater, watering with half-strength soluble cactus feed before sowing and a final spray with Chinosol. I find in the main that healthy seedlings grow fast enough to outcompete the moulds and algae.

[Chris L]

But then having sown the seed the first watering is by standing the pots in boiling water until it has soaked up.

Is this with plastic pots? I have some 2" BEF pots I bought 2nd hand and, as this method sounds appealing, I would rather not melt them when testing!

BEF pots are as indestructible as mealy bugs. They may go a little soft with boiled water but you won't melt them. Clean pots standing in clean tray.... boil kettle.... pour water into the tray and allow to cool.

*for safety I always place the tray on the floor where the tray cannot be kicked over or otherwise stood on... not recommended on a high up surface where it can be caught and spilled all over a person*

[Herts Mike]

Chris is spot on.