Articles
PROPAGATION HOUSES. A CRITICAL REVIEW
Plants differ in their ability to maintain turgidity.
F.i. a Pelargonium is able to root in the open air outdoors.
A Ligustrum cutting will wilt and die in a few hours.
Young cuttings with a minimum of epidermis and cuticula have a higher requirement for optimal rooting climate than the hardwood and semihardwood cuttings.
But if conditions are at optimum the young terminal cuttings will root within the shortest time.
In the process of rooting it is advisable to give also the tolerant species the best conditions.
Pelargonium will root under rough conditions within 4–6 weeks.
Under optimum conditions it will only take 16 days.
As water uptake through stem and epidermis is very restricted the water balance of the tissue must be achieved by a strict water economy which means a restriction of transpiration.
Transpiration from plants is strictly dependent on the difference in vapour pressure within the mesophyl and the surrounding air.
This difference is highly correlated with the difference in tissue temperature and air temperature – the Δ t value.
The Δ t is mainly a product of radiation and Δ t values of leaves are very closely correlated to net-radiation and therefore also to vapour pressure differences.
Even by placing the tissues in a saturated atmosphere there will be a difference and therefore a transpiration.
The boundary layer of air will approach the tissue temperature and dew points will be reached at cooler spaces.
To overcome the t and transpiration a reduction of radiation (light) is logical.
Shading has always been used.
Good reasons for shading can be found in the classical curve of photosynthesis in single leaves.
As many propagating spaces consist of a single layer or a rather thin canopy shading is justified for these conditions when light intensity is higher than 0.1–0.2 calories per cm-2 min-1.
Maintenance of high humidity in the surrounding air is naturally important and from early days the small lanterns or cloches were used for rooting.
When you look into French horticultural books propagating was synonym with use of glass cloches – and you will still see them by the thousands in certain districts.