If you sow the seeds during the summer months (July/August in middle Europe) there is no need of bottom heat. The best germination is at temperatures from 35° Celsius and higher. The humidity has to have a range of 50 -80%. If it´s too high the seeds might rot and get infected by fungus. If it´s too low there will be a delay in germination. To get compact seedling you should provide enough sunlight, otherwise the seedlings will grow too tall
