I've never seen perlite grow a crop of mold, but it sure can accumulate some nice algae (green), if the conditions are suitable.

I'd think that a top layer of perlite would scatter when you water. Have you had that problem?

...... and algae is far more prevalent when using organic sources of nutrition - fish and seaweed emulsions as well as fertilizers containing various forms of 'meals' are far more likely to produce a nice green algal bloom than soluble fertilizers. Sunlight also plays a considerable role. Over-watered plantings in full sun, especially those on which you might be using organic fertilizers, will find the algal populations petitioning the gnats to find a suitable way of saying thank you.

Al

lol...good one, Al, about the gnats. =) I am not using organic fertilizers, as some of you already know. The layer of perlite on the top does scatter a bit when the plants are watered, so I imagine it would eventually mix in to the rest of the potting medium somewhat. I do not have mold or algae on them currently at all. I just heard (I think on a thread on another forum) that someone said Perlite would grow algae or mold, so I wanted to find out for sure, especially if it would be harmful to the plants. Which kind of brings me to my next question. If algae does grow on it, would it be harmful to the veggie plants & if so, what should I do to prevent it? The plants have sunlight & a supplemental source of light, so i'm not worried about a lack of that. Also, I have the ceiling fan in the room on medium speed to help strengthen the plants, so air circulation shouldn't be an issue. Maybe am I just freaking out a bit about nothing? Anyway, as long as they will be ok for about 2 months in this potting mix, that's all I care about, as their final homes will be 10 gallon containers outside.

- Steve

If you have problems, it won't likely be from the algae, per se; rather, it will almost certainly be from the conditions that favored the algae growth in the first place (soggy soil). Lol - I actually caught what might be considered a Freudian Slip when I proofread. I had mistyped 'sorry soil' instead of 'soggy soil'. ;o)

Al

Haha...always the comedian, eh? ;-) Ok, I will keep an eye on the watering for the seedlings. They don't tend to do too well when overwatered in many ways, so I normally don't do so, but i'll keep an extra eye on the ones with the perlite layers. Thanks.

- Steve

Most of my tomato seedlings in the garage that I have recently moved outside had algae all over the top of the soil. I always keep a fan on them though. I noticed that once they went outside and the sun started to hit the top of the soil it started to go away. But the algae didn't bother the tomatoes at all. And yes the soil my seedlings are in is organic and does have fish and seaweed product in it.

Damon

The only real issue with algal growth is that it might slow down the exchange of soil and atmospheric gases as well as reduce the evaporation of moisture from the soil surface. But as long as you keep any growth broken up so that a crust can't form, it shouldn't be too much of a problem.

Same experience here. I've only seen a faint green algae form on the Perlite - never mold.
As Rhizo says, if the surface remains irregular/open, there shouldn't be a problem.

Josh

Great, thanks y'all. =) I'm not worried about it anymore & will probably continue with the thin layer of perlite on top of the pots cuz it looks cool!

- Steve