"The grass of the field, which is here today and tomorrow is thrown into the fire." Matthew 6:30

Almost every type of combustible matter was used for fuel. This would include: withered stalks of herb (grass);
wood (Isaiah 44:14-16);
charcoal (Jer.36:22; John 18:18);
shrubs (Ps.120:4);
thorn bushes (Eccl.7:6; Nahum 1:10);
weeds (Matt.13:40);
vines (Ez.15:4,6);
branch trimmings (John 15:6);
animal or even human dung (Ez.4:12); and
the bloodstained clothing of fallen warriors (Is.9:5)

Oil was used as fuel for lamps (Matt.25:3)

Coal was not known to the Hebrews.

Fuel is also used figuratively as a symbol of total destruction (Is.9:19; Ez.15:6; 21:32)

Jesus showed that God's amazing love was shown in clothing grass, destined to be burned as fuel, with beautiful flowers. This illustrated his even greater care for human beings.