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Everywhere, when discussing moving a plant to a new location/climate, people mention hardening it off. What does this term mean?

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'Hardening off' is the process by which plants which have been somewhere sheltered and warm are toughened up or gradually acclimatized to live happily somewhere less sheltered and less warm. Plants sold at garden centres which are under cover or inside will need hardening off, but those outside in the open won't need hardening. Seedlings you've grown yourself in controlled conditions, once potted on and growing strongly, will need to be hardened off before planting outside, as will any plant which has been kept inside or under cover over winter, and which you now might want to place back outside. It's important that the correct time for hardening off is chosen, depending on the plants involved - you'd only harden off a tender plant once you know that the temperatures outside are not currently, nor likely to become, cold enough to damage or kill the plant.

The procedure involved for hardening off is to choose a still, mild day, move the plants outside for three or four hours in the middle of the day, preferably not in full sun, then bring them back inside. Carry on doing this every day, extending the time they have outdoors over a week or so, until eventually, preferably on a mild night and in a reasonably sheltered spot, they are left outside all night. Once that's achieved safely, the plant is now hardened and can be either planted out, or its pot placed in a desirable position.

  • I like to buy from places that store plants outside so I can avoid this whole process. It can be tricky. If you leave them out on a bad day and you're at work you could lose everything. – NKY Homesteading Feb 17 '16 at 21:49
  • The trouble is, you don't know if they leave them outside all night - they may well be moving some inside or under cover after they close, depending on the time of year, and particularly with small plants. – Bamboo Feb 18 '16 at 14:04
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Basically plants from a nursery are "soft" i.e. they have been grown in very controlled, specific, routine conditions, and monitored regularly. When they are then taken out to site and planted, they are exposed to conditions that are completely foreign, be it aspect, exposure, moisture and soil composition etc. The 'Hardening Off' term applies to the period of "toughening" (read adjustment) that the plants go through during establishment at the new location. Some survive, some thrive, some die. Others may experience stunting or a slowing of growth which is then resumed when it has adjusted and comfortable in the new conditions.

The specifics obviously depend on the species and the location, but generally some things to take notice of are moisture, nutrients and damage. Keep it watered (but not too much), look for signs of nutrient deficiency and damage from the sun, frost or wind. Many trees and large shrubs will loose leaf when replanted, this should be temporary and discontinue after a short time if the conditions are suitable.

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