Hardening off plants is where tender plants such as seedlings, half hardy annuals, young plants and tender perennials are gradually exposed to cooler, more exposed conditions.
This enables the seedlings and plug plants to form a stronger epidermis, or outer layer (like our skin) so that they can better cope with temperature changes.
Hardening off seedlings, plug plants, young plants and tender perennials – how to do it
The whole hardening off process should take about two weeks, depending on weather conditions, for example, they should not be put outside when it is very windy.
For example, hardening off seedlings which have been sown into seed trays in the greenhouse.
The seedlings would be kept in the greenhouse until they reach a reasonable size, then they would be moved to a cold frame during the day. Initially, the pots of seedlings would be moved under cover overnight. They gradually move to a sheltered spot outside, during the day and then are planted out in the ground or in pots for a summer display.
The seedlings could be young vegetable plants, such as mange tout and runner beans, which are sown as seeds early under cover to increase the amount of vegetables and to spread the harvest over a number of weeks.
Half-hardy annuals, such as petunias and New Guinea impatiens also need hardening off in the same way as seedlings.
Sometimes half-hardy annuals are bought as plug plants rather than sown as seeds. These still need hardening off, as described above.
Tender perennials such as dahlias which are started into growth from tubers (roots) are also hardened off, generally during May, as they flower in late summer.
Mature tender perennials, such as orange trees are grown in pots so they can be given winter protection and put out on the terrace or patio over the summer. These plants also need to be hardened off, but as they are mature, the hardening off process does not take as long as seedlings and annuals.