We have needed to supplement with plenty of heat this spring. When we start our seedlings, the most important thing to keep warm is the roots. The system in this photo is of our greenhouse woodstove. Above it is a barrel filled with water. The water gets heated by the stove and the sun (which is why the barrel is painted black). The water is then circulated through white flexible pipes under the seed trays. The heat is needed for the roots to develop, so that when the weather gets warmer and sunnier the plants will quickly take off.