I keep my seeds (Hosta) in the dark until they germinate. It helps to keep algae down. Then when it starts to get too green on the surface, I sprinkle a little bit of dry seed starter mix (I use Fafard superfine) on the surface. That knocks it back. Algae can't grow without light.

The algae itself doesn't hurt your seedlings, but it is using up moisture and fertilizer. And it is contributing to the fungus gnat population. Fungus gnat larvae are feeding on the algae roots. I use Bt in my water as a means of keeping the fungus gnats down. That and yellow sticky squares does the trick.

As your seedlings get bigger, they shade the soil and the algae diminishes.

Good luck with your seed growing.

Steve

It isn't related to the mix used except in some secondary fashion as different mixes retain water differently.

Instead it is caused by the mix being too wet - either from the initial mixing with water or from later over-watering. The issue can be eliminated up front by making sure the potting mix is well wrung out prior to seeding or later by simply propping open one end of the dome for an hour or so a couple times a day. Or even easier, just removing it for an hour or so 2x a day.

As you mention, many don't use domes at all and it honestly works better. But it is a handy crutch for those who can't be home to monitor the flats frequently.

As to the damage it can cause - damp-off and death of the seedlings or stunted root development.

Give strong consideration to removing your domes and leaving them off until the developing algae disappears and the soil surface begins to dry out a bit. Then you can replace them if you wish and the remaining moisture will still be sufficient.

Dave

Thanks folks for your input!

Give strong consideration to removing your domes and leaving them off until the developing algae disappears and the soil surface begins to dry out a bit. Then you can replace them if you wish and the remaining moisture will still be sufficient.

Dave

As usual, spot on! I think a major part of my issues may be centered around dome use....... I'm retired, so I have time to experiment!

General potting mixes are meant for outdoors use where there is wind, sunshine ..etc. Obviously those mixes have livin organisms in them, that is algae, mold, etc. They also smell bad. If your indoor environment/soil is humid/damp and there is no or very little air circulation the algae , mold, fungi will grow. For this reason, it is better to use sterile starting mix or sterilize it yourself in overn, outdoor grill ...

Thank seysonn! Didn't realize all this was going on in my media..... Guess I need to start sterilizing my starter!

What role does Mycorise play in the growing media?

I have been using pro mix BX for 25 years for starting seeds and I keep my sown seeds under a humidity dome and never have a problem with mold or algae. I think you are making your pro mix BX too wet. I mix my pro mix with water before I plant. I use a tin foil roasting pan to mix my soil. You want it wet right through but not overly wet. I squeeze a handful as I'm mixing. When it is wet right through and I squeeze a handful I want some water to come out but not a lot. I plant my seeds and sprinkle the dry mix on top and then moisten it with a spray bottle. With the humidity lids are on that is enough moisture to keep the seeds moist for germinating and I do not water again until the seedlings are very dry.

A sprinkle of cinnamon over the mold or algae will take it away but if you've already planted lifting one end of the humidity lid will allow for some air circulation. You might also try putting a fan nearby

Note-- pro mix BX is made for hanging baskets and retains more moisture than pro mix HB. If you have a problem with the BX you might consider the HB. I like the pro mix BX because it retains more moisture and requires less watering but then I let my plants dry out too

Thanks dowlinggram for your ProMix insight! I've always thought that excess moisture may be a part of my problem..... I'm taking care not to over wet it when sowing seeds & have removed my domes in the germinator to see if that helps... I'm retired with time to monitor seedlings for adequate moisture.....

I'm fairly new to germinating annuals & I'm finding the process much harder than starting vegetables!

For this reason, it is better to use sterile starting mix or sterilize it yourself in overn, outdoor grill ...

ProMix, just like most all quality brand mixes, is sterile coming out of the bag or bale.

Your water is not and neither are your containers much less the air. There is no need to pre-sterilize it. The solution is with the moisture levels and the air circulation. Any kind of electric fan even remotely in the area will improve air circulation.

Dave

@ Any kind of electric fan even remotely in the area will improve air circulation.

Dave

Thanks again Dave! I've since placed a small 8" fan (on low speed) inside my germinator to move some indirect air around (sans domes on the flats) ...... This has made a huge difference! Just have to keep media moist.....

Nother 10 years & I'll have the hang of this stuff! The basics, anyway..........