Read More in The Sun Gro’er Issue 2/1 (2004)
With spring just around the corner and the recent introduction of the Sunshine Ellepot Propagation System, it seems like a good time to review the basic cultural requirements for successful vegetative propagation. The use of unrooted cuttings to produce annuals and herbaceous perennials is on the increase, partly because desirable species and cultivars from offshore production facilities can be delivered faster and for lower cost than ever before. And while the phrase ‘offshore production’ may touch off heated debate among protectionists and free-trade advocates, in this case cuttings that originate in the Southern Hemisphere while we shiver up here up North may actually
have a quality advantage. And that leads us to our first consideration, what constitutes quality in an unrooted cutting?
Much of the quality of an unrooted cutting goes back to the nutritional status of the stock plant from whence it came. Stock plants need enough nitrogen (N) to produce sufficient numbers of softwood cuttings but if too much N is supplied, carbohydrates (food) produced in the leaves during photosynthesis are utilized as fast as they are made to supply growth. The unrooted cutting needs a store of those carbohydrates to fuel root growth. This is where the aforementioned Southern Hemisphere advantage
comes into play. The higher light levels there during our darker winter months allow production of cuttings with higher carbohydrate reserves. On the other hand, a
cutting does need enough reserve N to sustain new growth through the initial period without roots. Adequate reserves of Ca and B are also important for the formation
of new cell walls.
No matter what the components used to create it, the ‘ideal’ medium performs three functions well. It has the strength to hold cuttings upright, it retains moisture well enough to keep cuttings turgid, and it drains well enough to provide oxygen for rooting. The latter two requirements usually are met in media that have 25-35% air-filled porosity. Container depth also has a direct influence on air-filled porosity. Although the percentage of pore space in a given medium is the same regardless of container height, there is a higher ratio of water to air in that pore space in shorter containers owing to less distance (height) from the ‘perched’ water table at the bottom of containers. Rooting percentages can be improved by not sticking cuttings too deeply to avoid this water-saturated zone. Other media requirements are pH about 6.0 and EC less than 0.75 mS/cm.
The basic requirements for successful vegetative propagation include a well-drained and easily sanitized site, with systems to provide bottom heat and mist, shade, and perhaps supplemental light. How the grower uses these systems to manipulate water, temperature, and light is the real key to success, so let’s examine each of these aspects more closely.
The goal is to minimize moisture stress of the cutting by maintaining humidity around the leaf to minimize transpiration. Misting also keeps the rooting medium moist but care must be taken to avoid overwatering. Too much water in the medium will reduce the level of oxygen supplied to roots. The level of misting should be the minimum needed to keep cuttings turgid. In early stages, mist 3-8 seconds every 5 minutes. Reduce misting as cuttings mature. If misting isn’t sufficient to keep the rooting medium moist, handwater occasionally.
One ‘trick’ that can be used to improve the turgidity of freshly stuck cuttings is to spray them with a wetting agent prior to placement under the mist. The wetting agent causes water to spread in a thin film over the leaf surface rather than bead up, thereby reducing transpiration. Try this out. The effect on freshly stuck cuttings can be quite startling when compared to the turgidity of cuttings left unsprayed.
Bottom heat is essential because root medium temperature is more critical than air temperature for rooting. Optimum root medium temperature is generally 72 to 75 F. Root temperatures can be reduced to as low as 60 F after roots form. Air temperatures do affect rooting and should be maintained at 65 to 75 F.
High light is not needed for rooting. Shade is used to lower light levels and thereby reduce transpiration. Not enough light, though, reduces rooting, so supplemental light may be necessary in winter. Light level recommendations will follow when we consider cultural specifics for the different stages of propagation.
No fertilizer should be applied until a visible callus (new tissue due to cell division around the stem wound) has formed, usually 5 to 10 days after sticking. After the callus
is visible, a weekly application of 50 to 75 ppm N as a complete fertilizer is sufficient. Once the root initials are formed, fertilization should be a weekly application of 100 ppm N. The low P and low NH4-N of Technigro 13-2-13 is ideal for this purpose. There is no need to apply fertilizer through the mist system. After rooted cuttings come off the mist but before they are transplanted is the time when plants are most susceptible to nutrient deficiencies, especially of N, Mg, and Fe. The nutrient status during this stage will have an effect on growth after transplant. During this stage, apply 125 to 250 ppm N as a complete fertilizer, and Epsom salts (Mg) and iron chelate as needed.
Propagation Requirements by Stage
Until now, I have referred to various stages of propagation without clearly defining them. Vegetative propagation, like plug propagation from seed, has well-defined stages that can be used in summarizing the specific cultural requirements for success. Jim Gibson of NCSU and Kim Williams of KSU provide the following recommendations set in stages:
- Description: sticking to swollen base
- Light: 500 to 1,500 footcandles
- Root-zone temperature: 68-80F
- Air temperature (night): 70-80F
- Mist: 3-8 secs every 5-10 min for 24 hrs; reduce to every 10 min after 3-4 days
- Media water status: keep moist
- Fertilization: none
- Description: callus to visible root initials
- Light: 1,300 to 1,800 footcandles
- Root-zone temperature: 68-73F
- Air temperature (night): 68-73F
- Mist: 3-5 secs every 10-20 min for 24 hrs; reduce to every 20-50 min close to Stage 3
- Media water status: moderate
- Fertilization: weekly 50-75 ppm N , low P , low N H 4
- Description: visible roots to significant root and shoot growth
- Light: 1,500 to 2,300 footcandles
- Root-zone temperature: 65-68F
- Air temperature (night): 65-68F
- Mist: discontinue; syringe if needed
- Media water status: reduce
- Fertilization: weekly 50-100 ppm N , low P , low N H 4
- Description: toning to transplant
- Light: 2,400 to 4,000 footcandles
- Root-zone temperature: 60-65F
- Air temperature (night): 58-65F
- Water: overhead or subirrigation
- Media water status: allow to dry between watering
- Fertilization: weekly 125-250 ppm N, low P, low NH4, Epsom salts, iron chelate
Final thoughts: Two other areas very important to success with vegetative propagation but beyond the scope of this short article are disease control, where sanitation is the key (disinfect floors, benches, trays; eliminate algae that leads to shore flies) and growth regulation (stock plants and cuttings). Other considerations include the handling
of unrooted cuttings upon receipt (open the boxes immediately; rehydrate cuttings under mist or in a cooler) and whether or not to use rooting hormones (woody plants show the most benefit). While whole books can be (and have been) written on the subject of vegetative propagation, my hope is that we’ve covered enough of the basic principles to ensure your success this spring.