Bottom heat is for germination only. It is NOT for growing on once germinated. It cooks the roots and the seedlings die.

The other likely explanation is over-watering. It kills more young seedlings than anything else. Young seedling roots need the air normally found in just barely moist soil more than they need water.

Remove the heat and cut way back on your watering and you should see much less loss.

Natural light this time of year is never enough for young seedlings. Not nearly enough intensity and not enough hours of light. Check out the FAQ here on the supplemental lighting recommended.

As to how long they can stay in plug trays, it all depends on the size of the cells in the plug tray - they come in many different sizes - but for the most part "plug trays" are for germination only too and transplanting into cell packs either at the cotyledon stage or at the longest, once the first true leaves begin to emerge is needed.

Hope this helps.

Dave

Thank you for the information. I have repotted almost all of the remaining seedlings into 3" pots and 6 packs and have moved them to our cold frame which is equipped with a light. We tested it before putting any plants in it and the temperature stays around 75 deg during the night (outside night temps in the 40's all this week). I'm retired so I can check it regularly during the day for ventilation. I know the air temp is not supposed to get over 75 or so. I also will pull back on the watering.

In your zone how long before you'll be able to transplant them to the garden? For me that won't be until the end of April so I don't want my transplants growing as fast as 75 degrees will produce. 75 degrees SOIL temp is great for germination but 75 degrees AIR temp is really too hot for growing on.

I maintain 60 deg. (65 max) for growing on air temps in the day and 50 degrees at night in the greenhouse. That way I get stocky transplants with strong stems rather than 3' tall leggy plants.

If it will be at least 35-45 days for you till transplant you may want to lower that night temp and and be sure to vent all day especially when the sun is out.

Dave

Thanks for the help Dave. Actually, after reading your answer last night we opted to turn off the lights and see what happened. So glad we did, especially after reading your second post this morning. We went out and checked the temp at 6am and it was 60 with no light. Our forecast for the week is pretty much the same until Saturday night, so we'll stick with the plan until then. We are usually able to plant outdoors mid to late March.

I check them continually during the day and usually have to start raising the top around 11-12 as its on the south side and shaded by trees until noon. Do you have a fertilizer recommendation? I don't want to burn them but I've been reading that they need some feeding soon. Thanks again for your help.

Fertilizer - most any well balanced one will do ok as long as you dilute the solution well before using it on seedlings. No more than 1/2 strength is the standard recommendation given no more often than weekly.

Dave

Thanks Dave. Happy gardening!!