Growing Delphinium from Seed in a Commercial Environment

The '7 Essential Points' in raising large quantities of our delphinium seed:

  • Always start with a sterile, well draining seed mix
  • Use a trichoderma fungus in the mix if possible
  • Avoid temperatures over 21°C in first week
  • Avoid temperatures over 16°C in second week
  • Temperatures below 16°C will slow germination
  • Never use cell trays containing more than 144 plugs, 72 is better
  • Darkness is not required but high humidity is

Our clients germinate our seed in a variety of conditions using a wide range of facilities. Detailed below is the method that works best for us and some methods used by others.

What works best for us:

We germinate and grow our seed in flats (for reasons of space) on a heated bench, then prick them out into cell trays or pots. The heated bench is filled with damp sand and covered with black polythene.

Seed storage –
Always store delphinium seed in the fridge.

Seed mix –
a fine ground bark, peat and fine pumice (natural exploded rock) mix in the ratio 2:1:1. This mix contains very little fertilizer but we also find that our standard potting mix, when sieved fine is ok too. The seed mix is a proprietary brand so it is readily available, holds moisture well but does not become waterlogged. The latter is important. The fibre contained in the mix has been composted to kill most pathogens and the pumice mix is land mined and cleaned. A Trichoderma fungus is added to prevent pathogens taking hold.

The use of the Trichoderma fungus has totally eliminated damping off problems, both above and below the surface. As a consequence our germination percentages are as high as possible. All viable seed grow.

Sowing -
Flats, 45 cm x 30 cm (18" x 12") are filled about 3.5cm (1½") deep with seed mix which is then lightly tamped down and thoroughly watered to leach any free salts away. The seed is sown, about 1000 per tray, covered lightly (0.25 to 0.5 cm (1/8th inch to 1/5th inch) with fine pumice and then watered lightly. The contrast between the dark potting mix and the light pumice cover makes it easy to check the depth of the covering pumice. The trays are then covered with 5 to10 (maybe) sheets (thickness) of clean newspaper to maintain humidity and capped with fibreglass cloth for insulation. We then water thoroughly over everything until we are sure the paper is damp right through. On the first sign of germination (after about 5 to 10 days according to variety) the paper must be removed. We leave the fibreglass cloth on in cold weather (if frosts likely) but otherwise take it off at the same time as the paper.

Temperature (first week) –
The heated bench is situated in a cool greenhouse out of direct sunlight. The temperature of the bench is kept at roughly 21°C (73°F) by use of a thermostat placed in the damp sand near the bench surface. Ambient temperatures vary according to season but rarely fall below 3°C at night and rarely exceed 25°C during the day. At the time of sowing most of our seed, minimum temperatures are about 5 to 10°C and daytime maximums about 15 to 20°C.

A thermometer is placed between the surface of the flats and the moist paper as a check on temperatures around the seed. These temperatures often fall to about 15°C on a cold night and rise to 22°C on a warm day. Temperatures above 22°C should be avoided and temperatures above 25°C can reduce germination significantly.

Temperature (after germination) –
Once the seed has begun to emerge we start to reduce temperatures. Taking off the paper and fibreglass cloth will help some, but in warm weather the bench thermostat may also need to be turned down. The object is to achieve a good growth rate but preserve plant health. We tend to leave the bench heating on for maybe a week after removing the coverings and then turn the bench off altogether or remove the flats to a cooler location where nights are maybe 5 to 10°C and days 15 to 20°C. The plants grow healthily in these temperatures.

Spray regimes – By using a trichoderma fungus in the mix we do not need to use any sprays whatsoever. However, it is a good idea to give the seedlings a sulphur spray as early as is convenient if powdery mildew is likely to be a problem.

Other Systems

Sowing into Cell Trays:
We do not normally sow directly into plugs but most propagators do. If sowing in plugs we use 72 cell plug trays. We do this because I believe we raise more plants than we would in smaller plugs. We therefore recommend using trays containing 72 plugs - certainly no more than 144. There are many reasons why:

  • disease control is better
  • hybrid delphiniums may not germinate evenly and if grown in small, tightly packed plugs the seedlings that emerge later will be crowded out
  • seedlings transplant better from larger plugs and they can be held for longer
  • we rarely have complaints about germination from clients using large plugs.

Sowing in Germination Rooms:
We are now hearing excellent reports from clients who use germination rooms where temperatures and humidity can be controlled accurately. In this situation a constant temperature of 16°C seems to give the best results. Once emergence in under way the seedlings are moved to growing rooms. Again, please use large plugs or sow in flats for later transplanting…. into large plugs.

Remember

  • Always start with a sterile, well draining seed mix
  • Use a trichoderma fungus in the mix if possible
  • Avoid temperatures over 21°C in first week
  • Avoid temperatures over 16°C in second week
  • Temperatures below 16°C will slow germination
  • Never use cell trays containing more than 144 plugs, 72 is better
  • Darkness is not required but high humidity is.
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Dowdeswell's Delphiniums - Wanganui, New Zealand