Plants and Christians are similar in their requirements for growth. 
Both need light, water, heat, and food to survive and to bear flowers and fruit. 

"There is a perennial cost to the Christian life. That is, it's not all cost. You see, some of us want to be in eternal summer and we want the Lord to shine on us everyday, and all days in the month in and out, and we want to have a lovely time full of fruitfulness never ceasing and so on, and the Lord says, 'Look here, I can’t do that. I’m not going to do that. There is a perennial cost. Now listen, the sharper the winter, the more fruitful the summer.'"