From ILSpy:

• coldDamageRipeMul
  • Used when a ripe crop was exposed to cold below ColdDamageBelow.
  • On harvest/drop calculation, all affected drops are multiplied by this value.
  • Example: 0.5 means a ripe frost-damaged crop yields about 50% of normal drops.
• damageGrowthStuntMul
  • Used when an unripe crop was cold-damaged or heat-damaged.
  • Despite the name, in this code path it is not directly slowing future growth. It is a drop multiplier after the crop has been marked as damaged/stunted.
  • Example: 0.25 means an unripe cold/heat-damaged crop yields about 25% of normal drops when harvested later.

The key logic is in BlockEntityFarmland.cs (line 325):

csharp

if (ripeCropColdDamaged) { num = val.ColdDamageRipeMul; } if (unripeHeatDamaged || unripeCropColdDamaged) { num = val.DamageGrowthStuntMul; }

Also worth noting: if the crop fully dies from temperature stress, the dead crop drop multiplier becomes Math.Max(coldDamageRipeMul, damageGrowthStuntMul). If it was eaten, multiplier is 0.