Heat mats are for germination only. The containers are removed from the heat mats as soon as the seedling breaks the soil surface. Algae growth is a result of overly wet soil and inadequate air circulation. Self-watering trays are notorious for causing it as the soil remains constantly too wet.

Here is a link to the same question and answers just a bit further down the page with details on what to do.

http://forums2.gardenweb.com/discussions/2935854/help-pro-mix-bx-and-green-algae?n=8

Hope this helps.

Dave

I keep some of my seedlings growing on heat mats for a while after they germinate because I start my seeds in the basement where air temps are very cool (around 50 degrees F) for most of the winter. I think many of them (depending on cultural preferences) appreciate the supplemental warmth until they have a few sets of true leaves. Today, it's almost at 60 F down there but I still have one tray of starts on a heat mat (mostly impatiens, basils, blue lace flower, begonia, and a few other things). I am guilty of overwatering (working on that - still a newbie) and I have poor air circulation so I have the algae issue as well. My understanding is that the algae itself isn't very problematic but it is indicative of conditions that can lead to damping off. Thankfully, I haven't had any problems so far with this (fingers crossed). I also keep the mat on a timer with the lights (12 hours on and 12 off) and have done this during the germination stage as well. Here is a photo of my set up today. The tray on the lower right is the one still on a heat mat. Despite the algae, everything appears healthy with the exception of a petunia starting to yellow probably due to overwatering.

<I keep some of my seedlings growing on heat mats for a while after they germinate because I start my seeds in the basement where air temps are very cool (around 50 degrees F) for most of the winter. I think many of them (depending on cultural preferences) appreciate the supplemental warmth until they have a few sets of true leaves. Today, it's almost at 60 F down there but I still have one tray of starts on a heat mat (mostly impatiens, basils, blue lace flower, begonia, and a few other things).>

Not needed and not really a good idea for several reasons. The ideal growing-on ambient air temp is 55-65 degrees. Once germinated the goal is slow top growth but rapid root development and higher soil temps, once germinated, only retards root development. You may 'think' it helps because you see faster top growth.

Plus while heat mats can increase soil temps by 10-15 degrees they don't increase ambient air temps by more than 2-4 degrees so it is wasted electricity.

Dave

Dave, thanks, I appreciate the input, but I do believe that growing some of my plants on the heat mats for a period of time post germination has helped somewhat. I can only go by what I have personally experienced, so I'm not saying it's the right approach for everyone, and certainly not for every plant. My snapdragons and Bells of Ireland have not received any supplemental heat, for example. Last year, I had miserable luck with my basil and cleome seedlings but not so this year. The soil temperature is the main variable that's different this year. Or maybe I just got a better batch of seeds - who knows? Maybe I'll have another opinion after a few more years of doing this, but I just wanted to share my personal experience and I seem to have had more positive results this year with this approach. I would not recommend keeping seedlings growing on a heat mat if they're in an environment with 55-65 ambient air temp.

I don't use heat mats at all, but use just a 60 watt light bulb under my seed trays. (I built a germination cabinet from a standard resin utility cabinet.) that generates enough heat for germination. If your basement is really cold you might need to use a 75 or even 100 watt bulb.