无霜期的英文：
frost-free season
frost-free growing season

frost是什么意思：
n. 严寒；霜；冷淡
v. 结霜（于）；冻坏

Frost is an enemy of flowers.
霜冻是花卉的大敌。

Frost caused the split in the rock.
霜冻使岩石裂了一条缝。

The crops were blasted by frost.
作物被霜冻毁了。

free是什么意思：
adj. 自由的；免费的；空闲的；任意的；免税的；游离的
adv. 免费地；自由地
v. 释放；免除；使自由，解放

Freedom is not free.
自由不是无偿的。

Be covered by free Medicare / be entitled to free medical care
享受公费医疗

It depends on free oxygen or air.
它依靠大量的氧或者空气。

season是什么意思：
n. 季节；时节；节期，时令
v. 给…调味；风干；（使）适应；（使）习惯

season cracking
风干裂缝, 应力腐蚀裂纹, 季裂

shooting season
狩猎期

This is not the season for harvest.
现在不是收获的季节。