A lot of people hit on important parts of the concept of hardening off. I think some of the unclarity comes from whether people are referencing leaves or branches. As mentioned above:
- Leaves are considered hardened off once they develop a waxy cuticle that protects from environmental conditions and somewhat slows transpiration. It helps protect against wind and sunburn. The leaves usually feel firmer and often look darker compared to brand new, tender leaves. There is often a mixture of new, tender growth, and hardened off growth EXCEPT in the spring (all new growth) and once growth stops (midsummer and fall).
- Stems/branches - new shoots are often soft, somewhat fleshy and green. As they harden off, or turn to semi-hardwood, they are a bit tougher. While not lignified (a protein that compiles on the outside, referred to as bark), they usually turn a darker color.
Reasons hardening off is referred to as an important transitional point in time:
- Energy: the tree invested its stored energy from the previous year into new shoots and leaves for the spring. After leafing out, the tree is now in an energy deficit. By the time the new leaves harden off, the tree is no longer is such as deficit - it has a new reserve of energy in case of something traumatic - whether environmental or human like pruning or defoliation. If something traumatic happens before the tree returns to an energy positive state (in which it has reserves), outcomes can be unpredictable: pruning with little back budding or alternatively tight, compact growth that seems ideal for refinement, yet a risk of losing branches or the tree dying. Fortunately, JM seem to handle losing leaves to frosts well.
- Pruning: some people say that the branch can die off if you prune it before it hardens off, while the buds are more consistently viable if you prune later, once they harden off (sometime in May or June usually, but that is tree and region dependent). I try to avoid pruning or damaging branches before they harden off, but when it does occur, I don't think I've typically observed the branch dying off / failing to put out a new shoot from a bud lower down.
- Wiring: semi-hardwood is, for many species, and ideal time to wire. Fresh, new growth can be easily damaged or broken off. Lignified, older growth can snap. Semi-hardwood tends to be durable yet still somewhat flexible. However, this is highly species and variety dependent.
Pinching is a common technique on JM that removes very new, tender growth. The point is to reduce elongation of internodes when refining a tree. In reference to the above bullet points... it does reduce energy and some experts say to refrain from doing it every year for several years straight, but your only removing a small amount of growth that the tree invested in. In terms of viability of buds, the whole point is that you stopped elongation and two new branches come from the spot where you pinched.
Lastly, its not black-and-white. The leaves are always in a transition from new to abscising. "Hardened off" is a point in between when the leaf is mature, has its cuticle and the stem is transitioning to semi-hardwood, but if you look along a branch in late May, you will likely have completely mature leaves at the base (oldest growth from this season) and completely new leaves at merging at the tip of the shoots.
