Hardening of tomato seedlings

Hardening of seedlings may sound cruel but it results in stronger healthier plants with higher yields. Hardened off plants are thicker, less succulent and more resistant to transplant damage and stress than non-hardened seedlings. Hardened off plants need less attention in the field resulting in more energy spent on other aspects of farming. Hardening off seedlings are more resistant to low temperatures¹, harsh weather, wind, high temperatures, drought[² and mechanical injury. It is also been said that they have better resistant to cutworm since the stems are harder to chew.

It is always recommended to transplant seedlings in the morning when temperatures are low and weather conditions favorable. We all know that is not always possible, especially if a large field must be planted. With hardened off seedlings a higher percentage of seedlings will survive this process even if they were transplanted later in the day.

How to harden of tomato seedlings

Hardening off seedlings is not difficult. It can be done in a week, but ideally it should be completed in two weeks. The seedlings are toughened by controlling the amount of water and fertilizer and lowering temperatures. Since it is very difficult to lower temperatures without expensive equipment, they most preferred method is reducing irrigation.

Needless to say, be careful of overhardening as it will stunt seedling growth and never recover. Stunted seedlings are weak and provide lower yields later in the season.

The first step of hardening is to decrease the amount of water and fertilizer by 50% in the second last week of seedling growth. In the last week, if the temperatures are not above 30°C, stop all irrigation and fertigation. In hot climates, only apply water during the hottest time of the day. Shading must also be removed so the seedlings can acclimatize to the sun’s direct radiation.

The transplants should be able to tolerate handling during transplanting and initial shock due to the altered harsh field environment.

1. Ghanbari, Fardin & Sayyari, Mohammad. (2018). Controlled drought stress affects the chilling-hardening capacity of tomato seedlings as indicated by changes in phenol metabolisms, antioxidant enzymes activity, osmolytes concentration and abscisic acid accumulation. Scientia Horticulturae. 229. 167-174. 10.1016/j.scienta.2017.10.009.
2. ZHANG, Shu-han & XU, Xue-feng & SUN, Ye-min & ZHANG, Jun-lian & LI, Chao-zhou. (2018). Influence of drought hardening on the resistance physiology of potato seedlings under drought stress. Journal of Integrative Agriculture. 17. 336-347. 10.1016/S2095-3119(17)61758-1.