JARS 42n4 - Implementing An Accelerated Growth Program

Implementing An Accelerated Growth Program
Joe Gidvilas, Jr.
East Granby, Connecticut

The commercial horticulture industry has an unfortunate basic problem - a very long production time from taking cuttings to getting saleable plants. In this article I would like to share with you some of the things I have done with an accelerated growth program to reduce the long production period and keep the plants growing without interruption. This was not a "scientifically controlled study", although I did experiment with different types of fertilizer and cultural methods.
We used tissue culture rhododendron liners which we brought in plus rooted cuttings of rhododendrons, leucothoe, kalmia, evergreen and Exbury azaleas from our propagation beds. These were placed into a soil mix containing three parts pine bark and two parts peat. The plants were potted into 3 and 4 inch square plastic pots, then put into a tray holding 18 pots. These were put on raised benches with poly tubes running underneath for heat.

For fertilization I experimented with two different formulations.

NUMBER ONE For each cubic yard of pine bark/peat soil mix:
3 lbs. Osmocote 17-6-10 plus minors (8-9 month formulation)
5 lbs. dolomite
lb. 0-20-0 superphosphate
2 oz. granular iron 40%

NUMBER TWO For each cubic yard of pine bark/peat soil mix:
3 lbs. Osmocote 18-6-12 (8-9 month formulation)
5 lbs. dolomite
1 lb. 0-20-0 superphosphate
2 oz. granular iron 40%

Also during this period I applied urea 46% at the rate of one teaspoon per gallon and muriate of potash 62% at the rate of teaspoon per gallon using a three gallon backpack sprayer once a week. The urea and potash were applied between 7 am and 10 am when the stomata were open. I used a fine sprayer mist for foliar absorption.
Soil samples were taken every three weeks. The pH was between 4.8 and 5.2. The N, P, and K were right on target. The problem of a soluble salt buildup was taken care of with a good leaching every two weeks. From my observation during the growing period, the fertilization program with Osmocote 17-6-10 plus minors was far superior to the Osmocote 18-6-12 as far as plant growth and quality were concerned.
Now, there were a few other things going on besides fertilization. These included the addition of CO 2 to the greenhouse environment, supplemental lighting during dark periods, temperature, ventilation, air circulation, pesticide control, watering and so on.

CO 2
Let's start first with CO 2 . I am not going to get into a long speech on why CO 2 is so important to plant growth, especially during the winter months, when you are into an accelerated growth program. During these months the greenhouses are closed during the day to conserve heat, especially here in northern Connecticut. Actually during the daylight hours CO 2 is removed from the air by plants through the process of photosynthesis and in a closed greenhouse the level continually drops and the rate of photosynthesis slows down until a point is reached at which growth stops. I carefully monitored the CO 2 level to assure good crop response, yet safe working conditions for the greenhouse employees.

Supplemental Lighting
Suspended three feet over the benches were two lines of lights spaced three feet apart using 75 watt clear bulbs with regular square pie tins for reflectors spaced every four feet. The lights, set on time clocks, went on each day at 5 pm and thereafter every 1/2 hour for five minutes then off until the next 1/2 hour. At 7 am the system shut off completely for the day.

The poly heating tubes that I mentioned at the beginning of this article run underneath the benches. All the benches had a sheet of plastic tacked down along the sides to the floor. The fronts and ends were also sealed. This was to keep the soil warm in order to activate the Osmocote fertilizer, which requires a soil temperature of 60 to 65 degrees for best results. The air temperature was 75 to 80 degrees on clear days. Soil temperature stayed the same in the night time hours, air temperature was 70 degrees.

I used a fan-poly tube winter cooling system which was operated manually. The reason I say manually is because it is important that before you ventilate you make sure the furnaces and the CO 2 heaters are off. In the case of the furnaces, you will actually suck the fumes right out of them and even after ventilating, you will have the smell of sulfur in the air.
In a 30 x 100 foot greenhouse I used two poly tubes; each one was six feet off the floor and ten feet from the side walls. The poly tubes have a hole every three feet along their length. I vented twice a day because of the humidity which can lead to all sorts of disease problems in the greenhouse. I've learned that for this size greenhouse and the fan-poly tube system it takes only five minutes to make one complete change of air. After venting the furnaces and the CO 2 burner go back on again.

Air Circulation
I used a horizontal air flow system. This is very important if you want to keep diseases down to a minimum. In a 30 x 100 foot greenhouse, I used four regular inexpensive square house fans that can be bought at any appliance store. On one side of the side, I mounted a fan five feet from the floor, eight feet from the sidewall and fifteen feet from the end of the house. A second fan was placed 40-50 feet from the end toward which the first fan was blowing. I reversed the procedure to mount the other two fans.

All pruning was done by hand with pruning shears. No growth regulators were used. Tissue culture plants tend to be leggy and they are pruned one-third back after potting. I believe that the right time to start shaping a plant is when the plant is starting to build its base of branches. I feel that if pruning time is missed on some species, they can never be brought back to a good branch structure.

During the accelerated growth program, all watering was done by hand. It actually didn't take that long.

I've found that by going on a preventative program, you can keep diseases and insects under control. Basically my program was as follows: Subdue 2/E or Aliette right after potting for the control of Phytophthora and Pythium. Thereafter I used Benlate and Dithane M-45 every three weeks along with Diazinon.

R. 'Yaku Princess', 1½ year old
'Yaku Princess', 1 year old using accelerated growth.
Photo by Joe Gidvilas, Jr.
R. 'Scintillation', 1½ year old
'Scintillation', 1 year old using
accelerated growth program.
Photo by Joe Gidvilas, Jr.

If you have the available space to do an accelerated growth program in the winter months, it is to your benefit. We are all striving to grow a saleable quality plant in the shortest possible time with the least amount of expense. The beauty part of this program is that you have an extra four or five months of growing.
Plants coming out of the program to be repotted into 1 or 2 gallon containers after all dangers of a frost are past kept right on growing with no shock to the plant. During the course of the growing season, with the right fertilization program, the plants really took off. The majority of plants were saleable 10-12" plus some 12-15". The investment was not great and paid for itself in a short time.
Some of the plants in the program were: 'Nova Zembla', 'Lee's Best Purple', 'Dora Amateis' and 'P.J.M.' from our propagation beds; also kalmia in variety. Tissue culture plants brought in included: 'Scintillation', 'P.J.M.', R. yakushimanum , R. mucronulatum , R. dauricum and evergreen plus deciduous azaleas.

Mr. Gidvilas, Connecticut Chapter member, has worked for 25 years in the nursery field as a production manager. His first interest is growing rhododendrons, but he is experienced in growing many other kinds of plants.