When he was in primary school/When he lived with his grandpa in the countryside.
题意:张超什么时候学会种菜的?
根据第三段第1-3行:The senior high school student said his love for planting was formed in primary school, when he lived with his grandpa in the countryside. He learned to grow vegetables from his grandpa, (这位高中生说,他对种植的热爱是在小学时和爷爷一起住在农村时形成的。他从爷爷那里学会了种菜)可知,他在小学时跟爷爷学种菜,那时他和爷爷住在农村。故答案为:When he was in primary school/When he lived with his grandpa in the countryside.