Grow Time (From Rooted Cutting)
A petunia in a quart container with one pinch per plant (ppp) takes five to six weeks of grow time. In a gallon container with one ppp, grow time is estimated at eight to nine weeks. Petunias in 12-inch baskets with four ppp take 10 to 11 weeks.
Apply 200 to 250 parts per million (ppm) nitrogen (N), using a calcium-magnesium formulation of a balanced fertilizer with low phosphorus (i.e. 13-2-13, 14-4-14 or 15-5-15 Cal-Mag) or a more acidic ammonium-containing fertilizer (i.e. 20-10-20 or 20-9-20) if keeping the pH low is a challenge. Electrical conductivity (EC) should be around 2.3 to 2.7 mS/cm (SME) with pH between 5.4 and 5.8. Supplemental iron-chelate (Sprint 138 or 330) as a drench (4 ounces/100 gallons) is recommended to avoid iron deficiency-related tip yellowing, especially under hot conditions and elevated pH (>6.0).
High-quality media with good porosity is critical to avoid root rot. Peat-based mixes, like Fafard 2 Mix or 1P Mix, or bark-based mixes, like Fafard 4P Mix or 3B Mix work well.
Petunias grow best when temperature is between 65°F and 75°F during the day and 55°F and 60°F at night. It can thrive in a colder environment, but this will delay flowering.
Generally only one pinch is needed for most pot sizes. Extra-large pots or baskets can be pinched twice. Use a viricide such as Virkon-S, RelyOn or Trisodium phosphate (TSP) viricides when pinching or trimming after at least every variety. An early application of Florel plant growth regulator at 350 to 500 parts per million (ppm) can be used to enhance branching on petunias.
While petunias are not as long-day (LD) sensitive as calibrachoa, some varieties are naturally later than others. It is always wise to provide night-interruption lighting on crops sold before April 1.
Plant Growth Regulators
To keep the plant size in check, use sprays of B-Nine plant growth regulator at 2,500 to 3,500 ppm or Sumagic plant growth regulator 15 to 20 ppm. Bonzi plant growth regulator drenches at 1 to 3 ppm also work nicely mid-season. Small-flowered Picnic types generally only need Bonzi drench rates of 1 ppm while more vigorous Whispers and Sanguna types typically require 2 to 3 ppm. As mentioned earlier, to enhance branching, some growers use an early Florel spray on baskets at 350 to 500 ppm.
Insects & Diseases
Thrips, budworm and leafminer, as well as various viruses, can cause problems. Flagship insecticide, Avid miticide/insecticide and Citation insecticide are the proven standards for a successful leafminer control program. Flagship, Avid and Conserve insecticides are also effective in a thrips management program. Conserve and Dipel insecticides will control most budworm issues. To minimize possible virus spread, use a viricide such as Virkon-S, RelyOn or Trisodium phosphate (TSP) viricides when pinching. Subdue Maxx, Medallion, Heritage and Daconil ULTREX fungicides will provide protection from root and crown diseases.