Forcing Hydrangeas
Friday, 27 February 2009 12:51
Allow newly arrived pre-finished plants to recover from shipping stress and to initiate active root growth (about 1-2 weeks) prior to transplanting into the final-sized pot. Hydrangea blooms are formed on the previous year's growth, so the flowers are already in place in the dormant buds. The ideal starting temperature for hydrangeas is a 60 to 62°F soil temperature supplied with bottom heat, while maintaining slightly cooler air temperatures (about 58°F). This allows root activity prior to bud opening on the shoots. Grow plants slightly on the "dry side" prior to transplanting to prevent root rot and to encourage root development. Avoid fertilizing until root activity has occurred.
One of the main problems with hydrangeas is poor root establishment which leads to water stress during late stages of forcing. To prevent this, several sources recommend slitting the root ball to form an X, when transplanting, opening the sections and placing in direct contact with the growing media in the pot.
To ensure clear pink or blue inflorescences, order cultivars programmed for the desired color and continue the color program throughout forcing. Fertilization practices during the previous summer growth phase influences coloration during forcing, and changing the color program during the forcing phase can result in shades of mauve tones.
Whether a hydrangea (excluding white cultivars) develops a pink or blue inflorescence is dependent on the presence and availability of aluminum. The absence of aluminum assures pink flowers; high availability of aluminum leads to blue flowers. By regulating aluminum, flower color can be controlled.
Pink Flowers Avoid supplying aluminum to plants and use fertilizers that do not contain aluminum. Use relatively high levels of phosphorus in the fertilizer program. Phosphorus antagonizes aluminum uptake and helps assure pink flowers. Rotating mono-ammonium phosphate (11-53-00) into the feed program will help raise phosphorus levels and help prevent aluminum uptake. An example feed program would be continuous feeding using 150 ppm nitrogen from 20-10-20 (10 oz/100 gal) rotated with 100 ppm nitrogen from 11-53-00 (18 oz/100 gal) every third feeding.
Try to maintain a substrate solution pH of 6.0 to 6.2; aluminum becomes more available at lower pH's. Be careful not to allow the pH to rise much above 6.4, or iron deficiency chlorosis will become a problem. Supply low to moderate levels of potassium. High levels of potassium tend to increase bluing of hydrangeas.
Blue Flowers Although dormant plants purchased as blues will have received aluminum sulfate prior to shipment, aluminum must also be supplied during the forcing period. Start drenching with aluminum sulfate immediately after transplanting. Apply 8 fl oz of drench per 6 inch pot using 10 lb aluminum sulfate per 100 gallons of water. Drenches should be applied to moist substrates only as drenching dry soil will result in damaged roots. Make applications at 10 to 14 day intervals. About 10 days after each application, measure the pH of the substrate. If the pH is higher than 5.6, another application of aluminum sulfate should be made. Continue this procedure throughout forcing. The aluminum sulfate not only supplies aluminum, it also maintains a low (5.2 to 5.5) pH in the substrate solution, desirable during forcing of blue hydrangeas. Use a phosphorus-free substrate for transplanting and use a fertilizer lacking phosphorus. Apply high levels of potassium for increased bluing. For example, apply 150 -200 ppm 21-0-0 nitrogen and 300 - 350 ppm 13-0-44 potassium at each irrigation.
Terminate fertilizer when flower buds begin to show color to increase the postharvest life of the flowers.
For more information
Commercial Hydrangea Forcing (PDF)by Douglas A. Bailey, North Carolina State University.
FloriCAST (short video) on fertilizing florist hydrangeas by Kimberly Williams, Kansas State University
Tina Smith, University of Massachusetts
