Focus On Fertility: Osteospermum
Osteospermum is a tender perennial, now classified outside of the dimorphotheca genus. In the more temperate climates, it can overwinter. However, when exposed to reoccurring frost or true freezing temperature, it will act as an annual. To homeowners, it goes by the common name of daisy or African daisy due to its flowers, which can vary in shape from small, large and spoon-like patterns. Producing this crop in a greenhouse can be done in many different container sizes to accommodate small-space containers, hanging baskets and also larger landscape plantings. Fertility requirements, however, remain the same.
A Strong Root System
At the youngest stage, hybrid osteospermum can best take up nutrients in the media when kept in an evenly moist environment at a pH between 5.8 and 6.5. Osteospermum have longer stems and need to support a large flower, so establishing a strong and healthy root system is key. Begin by applying fertilizer on a constant basis at a low rate of 75 and 100 ppm nitrogen. You can achieve the most control at this stage by using a water-soluble fertilizer, feeding until root saturation and allowing ample drying time for maximum drainage and disease prevention.
The Cool, Gray Days Of Winter
At the early stages of growth, especially when growing in the winter when light levels and day lengths are not as high or long as we would like, the nitrogen form in the fertilizer formula is crucial. It is recommended for some varieties to be grown longer at cooler temperatures. This is a benefit because it will save the cost of heating; however nutrient uptake can be slow and linger in the root zone.
Keep ammonium (NH4) nitrogen levels in the root zone less than 15 ppm by choosing high-nitrate-based (NO3) fertilizers, like 13-2-13, 15-5-15 and 15-0-15. These formulas are designed with a balance of nitrogen that is easy for the plant to absorb along with low but readily available phosphorus to encourage the growth and branching of a healthy root system. Early targets for media EC are between 1.2 and 1.5 mS/cm.
Water quality is a vital part in choosing the fertilizer that fits best for the remainder of the crop cycle. Osteospermum grown at cooler temperatures with lower phosphorous levels produce stocky plants with dense, dark-green leaves on each stem.
As nutrient demand ramps up, it is important to provide a balanced nutrient profile for strong, colorful flower display and on-time blooming. This includes providing all the major, minor and secondary nutrients from either your water source or your fertilizer selection. Calcium and magnesium may have to be supplemented to reach levels of at least 60 to 80 ppm calcium and 30 to 50 ppm magnesium.
Once a strong root system is established, growers with high alkalinity and high pH water sources can select a fertilizer that is potentially acidic, like 21-5-20 or 25-5-15. If it is necessary to supplement with magnesium, you can add Epsom salts directly to your stock tank. A conservative rate of 4 ounces per 100 gallons of water will achieve an increase of 30 ppm magnesium in the nutrient solution.
If your water source is more pure, you may need to increase or maintain your pH by adding calcium at each feeding. Complete formulas such as 13-2-13, 15-5-15 or 15-2-20 are very good options, since they add both calcium and magnesium in one balanced formula. However, they will raise your pH over time.
Neutral formulas such as 17-4-17 and 16-2-15 also add calcium and magnesium but are designed to keep the pH change in the root zone to a minimum. Compare these factors when selecting your fertilizer so you can provide a balanced nutrient profile as well as keep within the pH target values.
Feeding Enough To Ship Early
One of the benefits of these newer series is that they can be produced quickly as the weather warms and shipped with a nice display of early blooms. In order to achieve this, nutrient levels must be raised as the temperatures and light levels increase. At this stage, osteospermum are moderate to heavy feeders. The most efficient nutrient uptake will occur when the media stays in the targeted 5.8 to 6.5 range. When planted in the landscape, osteospermum can tolerate even higher pH levels, but when grown in the greenhouse, you will encounter micronutrient deficiencies if the root media pH is consistently above 6.2.
Gradually increase your overall fertilizer rates to 200 to 250 ppm as the plant grows to finish. All programs should still be using a lower phosphorus formula to prevent internode stretch. Monitor growing media to avoid a gradual rise in pH and EC, not to exceed a pH of 6.5 and EC of 2.5 mS/cm.
In order to promote blooms at the time of sale or shipping, you can add one booster feeding with a higher phosphorus fertilizer, such as 10-30-20. This is a timed application, usually 10 days before sale at a rate of 200 ppm N (600 ppm P2O5). This phosphorus boost, 2:1 potassium to nitrogen ratio along with the higher temperatures will encourage blooming, and the one-time application will reduce the tendency for the plants to become leggy.