Bottom heat

Providing bottom heat when propagating plants, particularly stem cuttings, can optimise the growth of roots, without heating (and therefore drying out) the tops of the plants. You can use it to create the ideal situation for plant growth – cool tops and warm bottoms!

The ideal bottom heat is generally around 18°C, but check exactly what temperature works best for the plant in question. Anything over 30°C can actually reduce the growth of roots.

Providing bottom heat can be done in a number of ways:

  • Purchase a propagator which has built in base heating. These can vary in size from windowsill propagators to quite large greenhouse ones.
  • Place soil warming cables in moist sand with at least 5cm sand above and below them. This is most effective for propagation if the cables are laid directly under an enclosed space, such as a propagation case. Cables are available for a range of uses, including propagation units, general greenhouse heating and cold frame warming, so ensure you purchase the right cable for the job.
  • Place an electric blanket (one made for the purpose, not a bed warming electric blanket) under a non-heated propagator.
  • Create a ‘hot bed’ in your greenhouse borders by layering fresh straw horse manure, soil and a dusting of lime (to neutralise the acidic manure). Create three levels of these layers and leave it for a couple of days to heat up before use.

Where finances allow you should aim to purchase a heating solution which has thermostatic control (obviously with the exception of the ‘hot bed’ solution) so that you can ensure the right level of heat is supplied to the plants and that you’re not spending money heating them unnecessarily.

When using a heated propagation unit it is important to keep the transparent cover over the plants to retain moisture.