This three-part series on hydrangea production will focus on 1. mineral nutrition 2. bluing of H. macrophylla and 3. effectively utilizing plant growth regulators to increase efficiency and plant marketability.
Nutrient management is an important part of ensuring that a hydrangea crop is quickly sellable. Hydrangea mineral nutrition, however, can be fickle, impacting aesthetic appearance, flowering, and plant growth and subsequent shoot elongation. Therefore, your goal is to a have full plant with green leaves that is preferably flowering, while also ensuring plant height does not stretch past quality guidelines or shipping targets. This overview of hydrangea mineral nutrient management will highlight regularly occurring nutrient deficiencies to help producers better understand the crop’s nutrient requirements as well as pitfalls to avoid when producing high quality hydrangeas quickly.
Hydrangea is the second most popular deciduous shrub in all U.S. horticultural markets. In 2014, more than 10 million plants were produced in more than 1,500 nurseries generating $91.2 million in shrub sales. There are numerous species of deciduous and evergreen hydrangea shrubs, small trees or climbers, which include approximately 1,000 cultivars and hybrids; however, only four hydrangea species are regularly produced in nursery cultivation: H. arborescens (smooth or mountain), H. macrophylla (bigleaf hydrangea, including var. normalis and subsp. serrata, which are used interchangeably with H. serrata), H. paniculata (panicle), and H. quercifolia (oakleaf).
In containers, hydrangeas grow best with a substrate pH of 5.5-6.5, with some species preferring or tolerating alkaline soils or substrates near or above pH 7.0. In containers, dolomitic lime is used to amend the substrate to supply both calcium (Ca) and magnesium (Mg) to the plant and obtain the desired pH. Micronutrients, in addition to sulfur (S), are also commonly added. Gypsum (CaSO4) can be used as an alternative to dolomite to supply Ca only, if no pH adjustment is desired and there is not an adequate amount of Ca in your water supply. Typically, a medium to high rate of controlled release fertilizer (CRF) is incorporated or top-dressed. Top dressed CRF may be applied to the surface of substrates at the time of planting and again the following summer depending on production cycle, weather (rain and temperature) and fertilizer longevity. Many fast-growing hydrangeas can develop yellow foliage (chlorosis) or grow slowly during the lag phase of release (the first 15 to 30 days after application) for some 12-14 month CRFs; therefore, growers can incorporate a relatively safe and quick release nitrogen (N) source, such as urea-formaldehyde, at the time of potting to ensure the plants have adequate N from the beginning of the crop cycle. Growers may also decide to fertigate for better control of growth or to supplement CRFs to maintain a desired nutrient level at critical points in crop production. Additionally, it has been reported growers can apply foliar urea in the fall, prior to leaf drop, to increase plant N content prior to dormancy and subsequent bud break in the spring. Urea is adsorbed as ammonium but can be lost via evaporation or volatilization up to 72 hours after application before being taken up by the plant. Foliar applications with excessive urea concentrations can result in leaf necrosis and premature leaf drop; however, this response is species- and cultivar-specific. Iron (Fe) deficiency, observed as interveinal chlorosis on newer foliage, has been reported anecdotally to be addressed with regular foliar applications or drenches of iron sulfate (FeSo4) or iron chelate. Substrate pH management, specifically, maintaining a pH < 6.0, is the easiest way to ensure Fe is readily available for crop uptake. Specialty fertilizers are available to aid in the bluing of bigleaf hydrangeas, which will be discussed in the second article in this series.
For field production, test soil early so that any lime, phosphate (P) or potash (K) can be broadcast and incorporated prior to planting. Signs of Fe and, to a lesser extent, boron (B), deficiency may develop at pH > 7.0. A typical recommendation for all shrubs is to apply no more than 50 lbs. N per acre (agricultural-grade N) in the spring, when soil temperatures are consistently 50 F or greater, and reapply again, if necessary, approximately three to five months later. Surface applications broadcast after the crop is planted are not always economical because N can volatilize if not incorporated. The per-acre rate can be used for side dressing, whether done by machine or hand. Excessive N rates will often promote foliage growth while reducing the number and size of inflorescences.
Mineral deficiency ID
Mineral nutrient deficiencies differ across taxa, and specific nutrient symptomology is lacking for the many hydrangea cultivars and hybrids now available in the industry. Below are generalities to help identify common nutrient deficiencies observed in Hydrangea sp. Mineral nutrient deficiencies are categorized into mobile (N, P, K, and Mg), partially mobile (S), or immobile mineral nutrients [Ca, Fe, manganese (Mn), and zinc (Zn)] mineral nutrients, referring to the ability to be translocated or moved within the plant once taken up from the soil or substrate. For example, mobile mineral nutrients can move from leaf to leaf via the phloem, whereas immobile mineral nutrients move primarily via water within the xylem. Deficiencies of mobile nutrients are usually observed at the base of the plant where the oldest foliage is, while deficiencies of immobile nutrients are usually observed at the top of the plant where there is new growth. Partially mobile nutrients are usually observed throughout the plant. Symptoms of nutrient deficiencies are described below.
Nitrogen (N) deficiency occurs on older, mature or lower leaves which may be chlorotic, uniformly light green or yellow, and may have brown/black or dying (necrotic) tips. New or young leaves may be smaller or have red margins (leaf edge), while some edges, stems, or bud scales could have a purple tint. Growth will appear reduced as a result of reduced shoot elongation and fewer developing buds.
Phosphorus (P) deficiency occurs in older, mature or lower leaves that may be slightly chlorotic, uniformly yellow and may have purple margins. Plants will have a stunted appearance as a result of shorter internodes and potentially decreased flower buds. New or young leaves may be smaller than normal and have a dark green or even blue-green coloration.
Potassium (K) deficiency can be observed in recently expanded or young leaves that appear dark green, lustrous (shiny), and narrower than expected. Shoots may have a rosette appearance due to being compact or having shorter internodes (distance between branches). Older, mature or lower leaves may first appear chlorotic (yellow), followed by rapid necrosis (brown/ black or dying tissue) along old leaf margins or as speckling.
Sulfur (S) deficiency in recently expanded or young leaves may be chlorotic (yellow), which can be pronounced on the leaf margins. Growth will appear reduced as a result of reduced shoot elongation and shorter internodes (distance between branches). Severe S deficiency can lead to defoliation or leaf drop. Sulfur deficiency can be confused easily with N deficiency (no photo).
Magnesium (Mg) deficiency first occurs in lower or older leaves which exhibit interveinal chlorosis (yellow leaf and green veins) and possibly red margins (leaf edge). The leaf margin can curl under (hooding) as deficiency worsens. Root growth may be reduced or shallow but appear healthy.
Calcium (Ca) deficiency occurs in recently expanded or young leaves as a light green, yellow to translucent appearance. Additionally, new growth can be necrotic, deformed or distorted. Roots may be densely branched, short and thick.
Iron (Fe) deficiency appears as interveinal chlorosis (yellowing) that becomes yellow or white, occurring on recently expanded or young leaves, and may have necrotic leaf areas along the margins. Iron deficiency can be confused with manganese (Mn) deficiency.
Manganese (Mn) deficiency presents itself as foliar interveinal chlorosis, yellow with green veins, which may occur on recently expanded or young leaves, eventually resulting in tan flecks on the leaf. Manganese deficiency can be confused easily with Fe deficiency.
Revisit your hydrangea mineral nutrition program to provide the correct available nutrients at the time the plant requires them. Additionally, send in a sample of your irrigation water to a local lab to determine the amount of dissolved mineral nutrients that might be contributing to overall plant health. Moreover, it is also a good time to measure the volume of water being applied to hydrangeas to ensure nutrients are remaining within the container while leaves stay hydrated. In the next article, we address various methods to maintain blue-flowered hydrangeas in container production at the time of sales.