Hi veggierosalie
The same thing occurs on most batches of perennial seedlings I have sown and left to germinate over a long period of time. I used to use a spray of water and vinegar with some success to keep it off. If I remember correctly, the moss died and was easy to remove that way. But I am not worried about the eye appeal of a batch of seedlings, as you must with bedding plants. And I have no idea how vinegar would affect your plants.
I sometimes sprinkle a layer of vermiculite, pearlite, or turface over the top of the mix to prevent moss formation and that will work if the layer is thick enough.
I have read that the moss is a naturally occuring, from the water we use, but don't honestly know. I do know that when I asked the makers of the soiless mix I then used about the occurrence, the company representatives purported that they had never heard of such a thing as moss formation.

I too have had the same problem. Not growing on the plant, but,on the surface of the potting mix. I figured it might be too much moisture or air circulation. Also, I have had a problem with a "rusty" color on the perilite from our hard well water. Does any one know if hard wated harms plants?

We have really hard well water too, with lots of rust...we don't even bother using it for the plants because when we were only growing house plants it would kill them! If you can get your water tested to see how much sodium, iron etc there is you may have a better idea if it is good enough to use or not. Now we only use treated water from the town pump (we live near a small town) or river water once the ice goes.

I think this moss must be due to moisture also, but if I let the seedlings dry out more they shrivel and die before the moss does...
cheers

I think it's easiest to prick or peal the moss off the soil. Throw it in the compost and add some fresh soil to the exposed roots.

When you say hard water, it normally means high content of calsium mainly, and some other minerals like magnesium. These actually enhances plant growth. I don't know much about iron, but our well water does have extremely high iron contents. Our water turns into literally brown overnight if we do not use water softner.(GW built a steel refinary not far from our house to produce cannon balls for the independence war.)

We sprinkle tons of this water (bypassing water softner) sprinkle over our flower and vegetable gardens, but there is absolutely no problem. NaCl is normally negative to plants though. If any hard water kills plants, it must have some other serious ingredients.

Regarding the moss problem, I might try oxidized water first, because it is a superb sterilizer. But, you will have to purchase a water electrolyzer, which is not cheap. If you want to know more about this, I will send you a brochure. (Please do not get me wrong. I am not trying to sell anything - it's a free brochure.)

Moss indicates that your water is neutral to acidic. Sprinkle existing moss with bone meal. Mix bonemeal into your planting mix next time. Try 1-2 cups per 5 gallons of potting mix.

Problem is worse if you don't let the surface dry between waterings. You want to get your seedlings tough anyway, so once they are 2" tall or so, let the top quarter inch dry before watering.

--
Sherwood Botsford
Sherwood's Forests
Warburg, Alberta T0C 2T0
http://www.sherwoods-forests.com
780-848-2548

peroxide will do the trick. I use pure peroxide (zerotol) through an injector at 50:1. The algae usually does not affect the plants, but will attract fungal gnats which are harmless.