After germination, most seedlings grow best if the air temperature is below 70 degrees F. If temperatures are too warm (over 75), the seedlings will grow too fast and get weak and leggy. Most seedlings grow fine in air temperatures as low as 50 degrees, as long as soil temperature is maintained at about 65 to 70.fter your seedlings are six to eight weeks old, you can transplant them into larger pots with a coarser growing medium, such as our Transplant Mix. If you wish, you can add up to 20 percent garden soil or sifted compost. A standard blend may contain 1/3 compost, 1/3 perlite or vermiculite, and 1/3 sphagnum moss.