Commercial Greenhouse Production

Scientific Name: Pelargonium hortorum

Family: (Geraniaceae)

Common Name: Seed geranium (annual)

Dr. J. Raymond Kessler, Jr.

Auburn University


Much of the production history of geraniums centers around the vegetatively propagated or cutting types of geraniums. However, in the early 1970s, researchers and breeders at The Pennsylvania State University developed the first true F1 hybrid seed geraniums. These are generally small plants than cutting geraniums, with small but more numerous flowers and a more self-branching growth habit. These characteristics of seed geraniums made them more ideally suited to market pack production and for traditional bedding-out in the landscape.


The inflorescence or flower stalk of a geranium is composed of many separate florets (flowers) in an

umber (flower cluster with pedicels arising from a single point). Flowers of most cutting geraniums are doubles, while those of seed geraniums are mostly singles. Cultivars of seed geranium are available in shades of pink, white, orange, scarlet, red, and bicolor. The foliage often has a zone or strip of anthocyanin in the middle, thus the term 'Zonal Geranium'. Current breeding goals emphasize reducing time to flower and increasing foliage zone intensity. Over 150 cultivars are currently available. The advent of seed geraniums may be one factor responsible for a leveling-off of cutting geranium sales in recent years.


Geranium seed are available as raw, refined, or primed seed. Germination medium for seed geraniums should be well-drained and well-aerated with a pH of 6.0-6.2 and a soluble salts <0.75 mmhos/cm. The pH is fairly critical because above 6.5 germination is inhibited and below 5.8 manganese and iron toxicity can become a problem. Seed do not require light to germinate and a light covering of vermiculite is often used to retain moisture. Germination temperature should be 75-78F in the first 3-5 days (stage 1), and reduced to 70-72F in stage 2. Temperatures above 78F during germination may cause seed dormancy. This is one of the few seed that seems to benefit from light fertilization after sowing. Calcium/potassium nitrate can be applied at 25 ppm nitrogen 1-3 days after sowing.


Temperature: Seed geraniums are generally grown at 62-65F night temperature and 68-75F day

temperature. Temperatures below 60F significantly reduce growth and delay flowering. Development of flowers from the visible bud stage to anthesis is governed by temperature. Increasing or decreasing temperature at the visible bud stage can be used to speed-up or slow-down finishing.

Photoperiod: Photoperiod significant response.

Light: Geraniums are high-light plants and the time from seeding to flower initiation and visible bud are directly related to light intensity. Geraniums are day-neutral plants and photoperiod is only important because plants under longer days receive more total light (cumulative light) than those under shorter days. Geraniums are must responsive to high light in terms reducing flowering time up to the 6th to 8th leaf stage which, for spring production, is a low light time of the year. High light increases the rate at which these first 6 to 8 leaves unfold and increases their size, thus the vegetative stage is shortened and flowering occurs earlier (reproductive stage). Therefore, it is essential to keep greenhouse glazing clean and free of shading compound.

Growing medium: Light, well-drained, peat-lite medium with a pH of 5.8-6.4 with an EC less then 1.5 mmhos/cm.

Fertilization: Apply a balanced fertilizer at 150-200 ppm nitrogen from a 20-10-20. Alternate with a calcium/potassium nitrate mix. Seed geranium grow best if the ammonium levels are low and calcium high. A once a month application of magnesium sulfate at 8 oz. / 100 gal. will supply needed magnesium. Note: Do not mix magnesium sulfate with calcium nitrate.

Seed geranium normal foliar analysis ranges
N 3.5-4.8
P 0.4-0.7
K 2.5-4.3
Ca 0.8-1.2
Mg 0.2-0.5
B 30-200
Cu 7-25
Fe 100-290
Mn 40-200
Zn 18-80

Growth Retardant: Chlormequat Chloride (Cycocel 1500 ppm spray) or Ancymidol (A-Rest 100 ppm spray) may be used to control plant height. However, Cycocel is preferred because it often reduces flowering time by a week or two.

Supplemental Light: Supplemental lighting from HID lamps significantly reduces time to flower, and ensures compact, sturdy seedlings with early leaf expansion and increased branching in seed geranium. Light geranium seedlings using 250-750 ft.c. for 16-18 hours/day starting at the first true leaf stage. The combination of Cycocel application with supplemental light accelerates flower more than the two alone.

Supplemental Carbon Dioxide: Rate of growth of seed geranium is accelerated by applying 800-1000 ppm supplemental carbon dioxide. Seedling are most responsive when young, so begin in stage 2 and continue throughout the plug stage or about 4 to 6 weeks.

Common Problems

Physiological: Petal shattering is caused by natural production of ethylene by the plant and is more of a problem for seed geranium than cutting geranium, though breeders are working on this problem.

Applications of silver thiosulfate (STS) have been used on developing flowers to suppress ethylene

production and delay shattering. However, plants must be treated for Pythium before applying STS. Low pH (< 5.8) should be a concern in seed geranium because manganese and iron toxicity can occur.

Pests: Pests are seldom a problem; spider mites, aphids, and mealybugs can be infest geraniums.

Diseases: Botrytis is particularly bad if spent flowers are not removed, it humidity is high, or if plants are spaced too close together. Rhizoctonia and Pythium can be serious problems causing seed rot, damping-off, and black leg.


Plants usually remain in the plug flats for about 6 weeks, then are transplanted to market packs for 4-5 weeks or 6-7 weeks in 4" pots. This can vary with the geographic location, cultivar, and time of year. In addition, treatments listed above such as Cycocel applications supplemental light and CO2 can reduce these times significantly. Therefore, records should be kept from year to year to make the proper adjustments to schedules.