The propagation process is littered with double-entendres, so prepare yourself.

‘Hardening off’ is the process of getting young plants, often raised in sheltered circumstances, used to the idea of being outside. This is especially true of any plants raised indoors, in a greenhouse or worst of all, in heated luxury in a propagator or similar.  These pampered plants are effectively too weak and feeble to go straight outside where they will be subject to the predations of the elements.

  • Direct sun
  • Wind
  • Rain
  • Cold
  • Temparature variations
  • Lower humidity

I think of hardening off as pre-season training for the plants.  They are healthy and in good condition generally, but not yet match-fit. The ‘training’ is a gradual introduction to the above elements that will toughen them up, harden them off even.  Depending on how pampered the plants are, this can be a surprisingly long process lasting several weeks. I’ll take it in stages.

Heated propagator or heated bench of some kind:

1) Open vents for a couple of hours each day, for a few days.

Why should I harden off plants
Heated but ventilated.

Unheated greenhouse staging:

1) Move plants to unheated staging in greenhouse, leave for 1 week.

How long does it take to harden off plants
Unheated staging. Shelves in this case.

2) Move closer to ventilation, eg open door, louvres, whatever you’ve got.  Couple of days here is fine.

3) Move plants to a sheltered spot outside for a while each day.  Not too exposed to wind, some shade is ideal. Ideally move them first on a dull day.  Start with an hour or two, increasing to daytime hours. If you’ve the space, also good to edge them into less sheltered spots over this period. When the time is up on a given day, put them back into the greenhouse where they came from.  Certainly do this before it gets too cool in the evening, or if you are expecting a late frost.

How long does it take to harden off plants?
Hiding in a reasonably sheltered spot.

3) after a week or so mucking about doing the okey-kokey with these plants they can go in a cold-frame.


1) You don’t have a cold-frame?  Make one immediately!  It can be unsophisticated, stack some bricks into a vague box shape and make a quick frame from battening, use some bubble wrap, polythene sheet if you have it and staple it to the frame.  Job done. Or just lean something approximating a cold-frame lid against a shed or the house.   Or cover plants with a couple of layers of horticultural fleece.

2) Once plants have gone through basic training above, the last stage is to expose to increasing amounts of elements in the cold-frame. They’ve already been through increasing amounts of daytime exposure, so in my head this is really about getting them used to night time temperatures.  The frame can be left open during the day, unless the weather is going to be truly unpleasant. First stage is to prop open the frame lid an inch or two in the evening and leave till beddtime, then close it. A day or two of this is fine, after which the lid can be left propped open over night.

Why do I need to harden off plants?
Easy does it. Propped open a little.

3) Over a period of a week or so, increase the amount of opening over night.  Prop it open more and more until it can be left fully open overnight. Do this over a few nights.

Almost fully trained.

After all that bother, if you haven’t given up entirely as it’s such a massive faff, the plants can either be planted out or moved out somewhere untidy and in the way until ready to go out. 

How to harden off plants.
Backlog of planting…

I don’t think this is an exact science, and frankly I never stick exactly to this regime – it’s a counsel of perfection. The process of moving things in and out of the greenhouse, for example, is tricky if you’re out at work all day – I just take them out in the morning and put them back in when I get home.  My favoured spot is under a tree so is reasonably sheltered if the weather turns while I’m at the office. Regardless of any compromises that might have to be made, it is certainly true that moving pampered plants straight outside is a recipe for disaster – they just aren’t up to it.  If they aren’t finished off by the elements, they are not strong enough to resist pests, iffy watering, diseases and the multitude of other afflictions. At the very least, a plant dumped unceremoniously into the great outdoors will have its growth checked.  Hardy plants acclimitise faster than tender plants.  Even if a little half-heartedly, and even if the full range of hardening-off stages is not available to you, find ways to ease your lovingly reared plants into their outdoor homes.

I’ll be back soon, with more propagation news.