Nutrient management for stock plants is similar to other long-term crops. We have worked with several leading stock plant growers both in the United States and overseas over the past decade. Together, we have found the keys for success are fairly straightforward: have an organized plan that includes media and fertilizer selection, organize crops into pH or EC groups, monitor nutrition regularly and ensure adequate levels of all nutrients are present in cuttings harvested from the stock.
1. Start with a quality growing medium. Some growers consistently produce excellent quality cuttings from stock plants grown in gravel, soil or locally produced compost to save costs. However, these locally produced substrates will often present the stock producer with challenges including inconsistent mixing, excess compaction or composting, limited root growth because of lack of aeration or excessive drying, and micronutrient toxicity (often manganese, depending on the rock type) when substrate pH decreases below acceptable levels (Figure 1).
We encourage cutting producers to run media trials using North American or European sphagnum peat-based substrates. In some cases, we have literally seen more than a doubling of cutting yield when several of these growers switched to course sphagnum peat-based substrates from a locally produced substrate.
Often, the plants grown in the sphagnum peat-based substrates have healthier roots and are better able to handle the stress of repeated cutting harvests.
When evaluating substrate costs, remember to include not just the price per pot, but also the value of the additional cuttings produced. We have found that it is easy to justify a higher cost and better quality substrate with even a slight yield increase (Figure 2).
2. Stock plants are long-term crops, so all essential nutrients must be provided on a regular basis. Nutrients must be provided either through (a) your water-soluble or controlled-release fertilizer, (b) growing medium components and preplant charge, © impurities in the irrigation water, (d) acid injection, or (e) periodic supplemental drenches. Typical nutrient solutions used in commercial stock plant production (water soluble fertilizers plus irrigation water impurities plus nutrients from acid fertilizers) are shown in Table 1.
If your nutrient solution has lower concentrations of key nutrients than those shown in Table 1, additional supplemental applications may be needed. In contrast, if a lab test of your irrigation water shows adequate levels of nutrients such as boron, calcium, or magnesium, these nutrients may not be needed from other fertilizer sources.
3. Cutting quality tends to be better when plants are grown with a majority of nitrogen in the nitrate form. Choose a fertilizer that contains at least 75 percent of nitrogen as nitrate. If media-pH tends to rise over time when using high-nitrate fertilizers, consider acid injection of the irrigation water.
4. Organize crops into their pH and electrical conductivity (EC) requirements rather than trying to treat every crop uniquely. Plants differ in (a) their ability to take up iron from the soil solution (iron efficiency) and (b) in their growth rates and root sensitivity to high salt levels. Table 2 shows grouping of a few example crops based on their requirements of both substrate pH and EC.
5. Set up a regular monitoring program that includes:
- A pre-season complete media and water analysis. Also, check any new batches of growing media when replacing your stock plants during the season.
- Weekly checks on fertilizer injectors to ensure the desired fertilizer concentration is being delivered.
- Check water pH and EC at the same time to ensure your irrigation water is not changing.
- Weekly or biweekly pH and EC testing of growing media. Choose whatever method (1:2, pour-through, SME) works for you, use calibrated meters and a consistent protocol, and select key crops (financially important and prone to problems) rather than trying to test every crop.
- Test the tissue nutrient levels of cuttings from key crops starting two weeks before harvest. The ideal sample for cutting production is 2 cups (enough for complete analysis including nitrogen) of whole cuttings (so the sample represents the product you will harvest), taken from multiple plants (to be representative), which are washed in distilled water (to remove fertilizer salts) and then air dried (so they will not rot in transit) before sending to the lab.
6. Have an organized plan in which the grower reviews nutrient test results each week, and decides on corrections. Testing without action is not management–it is busy work. Decide what actions will take place when common nutritional problems occur. For example, what do you do when (1) the media-EC is low (we suggest fertilizer drenches with 400 ppm N), (2) media-EC is high (we suggest leach with 50-plus percent of the applied volume), (3) media-pH is low (we suggest switching to high-nitrate fertilizer, stop acid injection, or drench with potassium bicarbonate or flowable lime), or (4) media-pH is high (we suggest injecting acid in irrigation water to a pH of 5.0 and temporarily shifting to a higher-ammoniacal based fertilizers).
7. Have specific actions ready when tissue levels or plant symptoms show a deficiency. There is insufficient space here to discuss correcting all potential problems. However, a few words of caution:
- If the stock plants are healthy and cuttings are performing well in propagation, do not over-react to lab results. Compare tissue levels against visual root health, media-pH and EC tests, and current fertilizer rates and formulations to come up with a common-sense corrective strategy. For example, deficiency symptoms may have occurred because of root disease rather than lack of fertilizer.
- Not all “high” or “low” levels in laboratory tissue nutrient reports represent a problem. The deficiency problems we have seen most often in herbaceous cuttings are nitrogen, phosphorus, calcium, magnesium, iron, manganese, and boron deficiencies (Figure 3). Deficiencies in other nutrients are possible but rare. Tissue analyses only represent results at one moment in time on a small sample, and many labs do not have well-established ideal tissue ranges for your specific crop.
- Ideally, your tissue nutrient levels should be in the upper part of the acceptable range for these nutrients, because tissue nutrient concentration typically declines during propagation while cuttings begin to grow with limited root systems.
8. Some growers add up to 2 ppm iron from iron-DTPA or iron-EDDHA chelate on a constant basis to keep iron-inefficient crops dark green and avoid iron deficiency if media-pH increases above 6.2. This is a good strategy. However, do not supplement high levels of iron to iron-efficient plants or toxicity symptoms are likely if pH drops below 6.0. Do not overdo supplementing any nutrient and maintain media-pH at a moderate level. Otherwise, toxicities (Figure 4) or imbalances can occur.
In conclusion, developing an overall nutrient management strategy for your stock plants will result in fewer problems during propagation of those cuttings.