Learn how to feed this valuable crop effectively and be aware of the symptoms of nutrient deficiencies.
This three-part series on hydrangea production will focus on 1. mineral nutrition 2. bluing of H. macrophylla and 3. using plant growth regulators wisely to boost productivity and plant marketability
A crucial component of making sure a hydrangea crop is quickly marketable is nutrient management. However, hydrangea mineral nutrition can be erratic, affecting the aesthetic appeal, flowering, plant growth, and ensuing shoot elongation. Your objective is to have a full plant with green leaves that is ideally in bloom while also making sure that the plant’s height does not exceed quality standards or shipping targets. In order to help growers better understand the crop’s nutrient requirements, this overview of hydrangea mineral nutrient management will highlight frequently occurring nutrient deficiencies and highlight pitfalls to avoid when producing high-quality hydrangeas quickly.
The second-most popular deciduous shrub in the U.S. is the hydrangea. S. horticultural markets. More than 1,500 nurseries produced more than 10 million plants in 2014, generating $91 million. 2 million in shrub sales. There are roughly 1,000 cultivars and hybrids of the many species of deciduous and evergreen hydrangea shrubs, small trees, or climbers; however, only four of these hydrangea species are regularly cultivated in nurseries: 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).
Hydrangeas grow best in containers when the substrate pH is around 5. 5-6. 5, with some species requiring or tolerating alkaline substrates or soils with a pH of 7 or higher. 0. Dolomitic lime is used to adjust the substrate in containers, supplying the plant with calcium (Ca) and magnesium (Mg), and achieving the desired pH. Micronutrients, in addition to sulfur (S), are also commonly added. If no pH adjustment is needed and there is insufficient calcium in your water supply and dolomite is not an option, gypsum (CaSO4) can be used as a substitute. Controlled release fertilizer (CRF) is typically incorporated or top-dressed at a medium to high rate. Depending on the production cycle, the weather (rain and temperature), and the longevity of the fertilizer, top dressed CRF may be applied to the surface of substrates when planting and once more the following summer. In order to ensure that plants have enough nitrogen (N) from the start of the crop cycle, growers can incorporate a relatively safe and quick release nitrogen (N) source, such as urea-formaldehyde, at the time of potting. This will prevent many fast-growing hydrangeas from developing yellow foliage (chlorosis) or from growing slowly during the lag phase of release (the first 15 to 30 days after application) for some 12-14 month CRFs. At crucial times in crop production, growers may also choose to fertigate for better growth control or to supplement CRFs to maintain a desired nutrient level. Furthermore, according to reports, growers can apply foliar urea in the fall before leaves fall to increase plant N content before dormancy and subsequent bud break in the spring. Adsorbed as ammonium, urea can evaporate or volatilize for up to 72 hours after application before the plant absorbs it. Leaf necrosis and premature leaf fall can occur as a result of foliar applications with excessive urea concentrations, but this reaction depends on the species and cultivar. Anecdotally, it has been stated that regular foliar applications or drenches of iron sulfate (FeSo4) or iron chelate can address iron (Fe) deficiency, which is seen as interveinal chlorosis on younger foliage. Substrate pH management, specifically, maintaining a pH < 6. The simplest way to guarantee Fe is easily accessible for crop uptake is 0. Bigleaf hydrangeas can bluish with the help of specialty fertilizers, which will be covered in the next article in this series.
Test the soil early for field production so that any needed lime, phosphate, or potash (K) can be broadcast and incorporated before planting At pH, Fe and, to a lesser extent, boron (B), deficiency symptoms may manifest. 0. Applying no more than 50 lbs. of pressure is generally advised for all shrubs. When soil temperatures are consistently 50 F or higher in the spring, apply N per acre (agricultural grade N) and repeat if necessary three to five months later. Because N can volatilize if not incorporated, surface applications broadcast after the crop is planted are not always cost-effective. For side dressing, whether done manually or by a machine, the per-acre rate can be used. In many cases, high N rates will encourage the growth of the foliage while lowering the number and size of inflorescences.
Specific nutrient symptomology is lacking for the numerous hydrangea cultivars and hybrids currently available in the market, and mineral nutrient deficiencies vary across taxa. The generalizations listed below can be used to spot typical nutrient deficiencies seen in Hydrangea species. Mineral nutrient deficiencies are divided into three categories: mobile (N, P, K, and Mg), partially mobile (S), and immobile (Ca, Fe, Manganese (Mn), and Zinc (Zn)) mineral nutrients. The ability to be translocated or moved within the plant after being absorbed from the soil or substrate is what determines which category a mineral nutrient is in. For instance, immobile mineral nutrients primarily travel through water in the xylem while mobile mineral nutrients can travel from leaf to leaf via the phloem. While deficiencies of immobile nutrients are typically found at the top of the plant, where there is new growth, deficiencies of mobile nutrients are typically found at the base of the plant, where the oldest foliage is. Partially mobile nutrients are usually observed throughout the plant. Symptoms of nutrient deficiencies are described below.
A lack of nitrogen (N) causes older, mature, or lower leaves to become chlorotic, uniformly light green or yellow, and/or to develop dying (necrotic) or brown/black tips. While some edges, stems, or bud scales may have a purple hue, new or young leaves may be smaller or have red leaf edges. As a result of decreased shoot elongation and fewer buds sprouting, growth will appear to be slower.
Older, mature, or lower leaves that may be slightly chlorotic, uniformly yellow, or with purple margins are indicative of a phosphorus (P) deficiency. Because of shorter internodes and possibly fewer flower buds, plants will appear stunted. Young or newly formed leaves may be smaller than usual and colored in a dark green or even blue-green hue.
Recently expanded or young leaves that are dark green, lustrous (shiny), and narrower than expected are signs of potassium (K) deficiency. Because they are compact or have shorter internodes (the distance between branches), shoots may resemble a rosette. Older, mature, or lower leaves may initially appear yellowed (chlorotic), then quickly turn brown or black (necrosis) along the edges of the leaves or as speckling.
Young or recently expanded leaves that lack sulfur (S) may be chlorotic (yellow), which can be particularly noticeable on the leaf margins. Reduced shoot elongation and shorter internodes (the distance between branches) will make growth appear to be slower. Severe S deficiency can lead to defoliation or leaf drop. Lack of a photo makes it easy to confuse sulfur deficiency with N deficiency.
Lower or older leaves with interveinal chlorosis (yellow leaf and green veins) and possibly red margins (leaf edge) are the first to experience magnesium (Mg) deficiency. The leaf margin can curl under (hooding) as deficiency worsens. Root growth may be reduced or shallow but appear healthy.
In newly expanded or young leaves, calcium (Ca) deficiency manifests as a light green, yellow, or translucent appearance. Additionally, new growth can be necrotic, deformed or distorted. Roots may be densely branched, short and thick.
Interveinal chlorosis (yellowing), which develops into yellow or white, appears on recently expanded or young leaves and may be accompanied by necrotic leaf areas along the margins in cases of iron deficiency (Fe). Iron deficiency can be confused with manganese (Mn) deficiency.
Foliar interveinal chlorosis, which is yellow with green veins and may appear on recently expanded or young leaves, is a symptom of manganese (Mn) deficiency and eventually leads to tan flecks on the leaf. Manganese deficiency can be confused easily with Fe deficiency.
Review your hydrangea mineral nutrition plan to make sure you’re giving the plant the right nutrients when it needs them. Send a sample of your irrigation water to a nearby lab as well to find out how much dissolved mineral nutrient content may be affecting the health of your plants in general. Additionally, now is a good time to gauge the amount of water being applied to hydrangeas to make sure nutrients are still contained in the container and the leaves are kept moist. We discuss various strategies to keep blue-flowered hydrangeas in container production at the time of sales in the following article.
Dr. Jim Owen (jsowen@vt. edu) works at the Hampton Roads Agricultural Research and Extension Center in Virginia Beach, where she is an Associate Professor of Horticulture at Virginia Tech. Dr. Anthony LeBude (avlebude@ncsu. edu) teaches horticultural science at North Carolina State University in Mills River, North Carolina, at the Mountain Horticultural Crops Research and Extension Center. C. Dr. Amy Fulcher (afulcher@utk. edu) is a member of the University of Tennessee’s Department of Plant Sciences in Knoxville, Tennessee.
The following extension publications contain this knowledge along with more: “Hydrangea Production: Cultivar Selection and General Practices to Consider When Propagating and Growing Hydrangea” (http://bit.ly/HydrangeaProduction); ly/PB1840A), “Hydrangea Production: Species-Specific Production Guide” (http://bit. ly/PB1840B), and the hydrangea chapter in “IPM Shrub Production” (http://bit. ly/2sPSkUq).
Preventing iron deficiency-chlorosis on hydrangeas
Although natural soil typically contains enough iron, hydrangeas frequently have trouble absorbing enough of it. This occurs when the soil pH is too high. The pH range for hydrangeas’ ideal soil is between 4 and 5. 5. They rely on the ability to take up iron directly from the soil in a 2-valent form (Fe 2), unlike other plants. However, divalent iron is almost exclusively found in acidic soils where iron has undergone this level of oxidation. Hydrangeas are certain to experience what is known as a “relative iron deficiency” in calcareous soils, where iron is present but cannot be absorbed by the plant.
Observe that the pH of the soil has an impact on the color of the flowers in plants. For instance, blue hydrangeas require a pH of 4 to 4. 5 to be able to absorb sufficient aluminum for the development of blue flowers
The best way to stop hydrangeas from developing iron deficiency-chlorosis is to plant them in suitable acidic or ericaceous soil. Mix some peat or reduced-peat ericaceous compost into the planting bed, and check the pH level once a year. This is essential because the compost mixture around the plants will eventually affect the soil’s pH level, and the pH level might even rise again.
Perform a pH test to determine the soil’s pH level. You can purchase testing kits from a variety of suppliers online or in some garden centers and hardware stores. If the measured pH is too high, ericaceous compost or lime-free peat can be used to lower it. It is best to start growing potted hydrangeas in ericaceous (acidic) soil.
Fertilizer Will Change the Color of Blooms
Not true. See the next section on pH.
This statement is definitely not true for most hydrangea. The majority of them are white, but as they get older, they start to turn green or pink. It is only the H. macrophylla and H. serrata that show blue or pink depending on soil conditions. H. paniculata and H. arboescens will not change color with a change in pH.
A change in pH does not cause the color to transition from blue to pink. No matter what the pH is, you will never have blue flowers if your soil is devoid of aluminum. The soil’s aluminum content is what turns the flower blue.
We need to examine what transpires in soil when pH changes in order to better understand why so many people advocate altering the pH of soil in order to cause a color change. The soil tightly holds the aluminum molecules in alkaline conditions, preventing the plant roots from accessing them. The flowers are pink as if there is no aluminum in the soil (ref 3).
As the pH in soil becomes more acidic (below 5. 5) The soil releases aluminum, making it available to the plant. Aluminum is taken up by the plant, transported to the flowers, and turns them blue. Your flowers may be receiving some aluminum if they appear purple, but not enough to produce the true blue color that so many people desire.
Aluminum will become tangled in too much phosphorus, making it unavailable to plants (ref 2).
This is only applicable to the macrophylla species, and it is only partially true.
Alkaline soils cannot produce blue flowers by incorporating aluminum into them. Since plants cannot use aluminum, it will simply be absorbed by the soil, as was discussed in the previous section.
Some soil, like potting mix, may not have enough aluminum. If aluminum isn’t added to the soil, you won’t get blue flowers even when the soil becomes acidic. In this case the statement is true.
The symptoms of chlorosis include yellowing of the leaves and veins. This is not due to an iron deficiency. Interveinal chlorosis, or the yellowing of the leaf spaces between the veins, is what people mean when they talk about an iron deficiency. This can be seen on the lower leaves in this picture quite clearly.
This symptom can be brought on by a lack of iron, but it can also be brought on by a lack of other nutrients. Even high levels of nutrients like phosphate, manganese, copper, or zinc may contribute to it. Iron is changed into an insoluble form in alkaline conditions, making it unavailable to plants. In conditions where they can more easily access iron, hydrangea prefer a slightly acidic environment.
Iron in the soil won’t stop hydrangea interveinal chlorosis if the issue is high levels of other nutrients or a high pH. Visit Chlorosis in Plants – Is It Iron Deficiency to learn more about this.
My soil is slightly alkaline at pH 7. 4. In the spring and the beginning of the summer, hydrangea typically have green leaves, but as the summer goes on, the lower leaves begin to exhibit interveinal chlorosis. I believe the plant is lacking in iron and is transferring it from older leaves to the newer top leaves. I can’t prove this, but plants do transfer some nutrients in this manner. How do I deal with this problem? I do nothing. The plants grow just fine and bloom well. Why solve a problem that does not exist.
Hardiness zones for hydrangea can be very misleading. They almost always indicate the hardiness of the plant. Many H. Zone 5 is a hardy area where macrophylla are sold frequently. The stems and buds are not as resilient as the roots, which is a problem. In zone 5, plants hardly ever flower because they do so on last year’s growth, which dies in the winter.
The same holds true for the oakleaf hydrangea. Despite rarely blooming in zone 5, it is quite hardy there.
When purchasing a plant that blooms on old wood, such as paniculata and arborescens varieties, be sure to check the bud hardiness because otherwise you might never see flowers on the shrub.
Hydrangeas Turn Blue if You Bury a Nail Under Them
People assert that numerous items, such as razor blades, hairpins, and copper pennies, have the ability to turn hydrangeas blue. These items must be made of aluminum, or at the very least contain a significant amount of aluminum, in order to have any chance of functioning. Most nails are made of steel, not aluminum.
In addition, the object would have to corrode quickly enough for the aluminum to enter the ground. Aluminum is very stable and corrodes slowly. Aluminum nails exist mostly because they don’t corrode.
Although choosing copper seems strange, there is some sound chemistry at play. Copper would combine with hydroxyl ions, bringing the pH of the soil down and releasing aluminum. The issue is that copper corrodes very slowly and is extremely stable. Copper pennies do not change soil pH.
It is better to use aluminum sulfate. The addition of aluminum ensures that there is enough aluminum in the soil while the sulfate will acidify the soil. Since aluminum is toxic to many plants, I personally would not use aluminum sulfate.