Hardening Transplants

When both the calendar and thermometer indicate that it is time to plant outdoors in Pennsylvania, I have trouble controlling my enthusiasm. I have rushed to plant both warm-season vegetables and tender annuals too quickly without allowing the plants to harden. This has resulted in wilting, brown leaf margins, slowed growth, and sometimes plant death. Here is an explanation of what hardening entails, why it is important, and how to do it successfully.

What is Hardening?

Hardening, or "hardening off," is the process of allowing a plant to transition from a protected indoor or greenhouse environment to the harsh outdoor conditions of fluctuating spring temperatures, wind, and full sun exposure. A gradual introduction of these outdoor stresses will cause the plant to accumulate carbohydrates, to trigger more root development, to reduce the amount of freeze-prone water in the plant, and to actually thicken its cell walls. Plant growth will change from soft and supple to much firmer and harder.

Hardening Timetable

• Start the process of moving plants outdoors about two weeks before the weather will be favorable enough for the particular plant to live outdoors.
• Check seed package instructions or inquire where you purchase seedlings as to when the plant can tolerate outdoor conditions. (Keep in mind that air temperature is often warmer than soil temperature.)
• This Vegetable Planting and Transplanting Guide provides guidance for when some popular crops can be planted outdoors.

Hardening Process

• When temperatures are at least 45-50^ᵒF, move plants outdoors to a shady, protected spot.
• Initially, place in the shaded, sheltered location for two to three hours.
• Gradually increase the amount of sunlight the plants receive over the two-week period. The last day or two, the plants can spend 24 hours outside.
• Reduce the amount of water plants receive, but do not allow them to wilt.
• Avoid placing seedlings outdoors on windy days.
• Cold frames are excellent places to harden plants, but another spot that provides protection, such as a porch, will work.
• Pay attention to the weather forecast; if temperatures will fall below 45^ᵒF, be prepared to bring the plants inside.

Keep in mind that the overall goal of hardening is to slow the growth of the plants to allow them to adjust to a change in conditions. After proper hardening, even warmth-loving vegetables, such as tomatoes, can withstand an unexpected dip in spring temperatures.