Plugs are rapidly replacing fresh-dug bare-root and cold-stored frigo plants as transplants for strawberry (Fragaria × ananassa) production worldwide. Plugs have many advantages over these other types of propagules. They are grown in controlled environments (greenhouses, tunnels) in less time than field produced bare-root transplants, and are not exposed to soilborne pathogens. Plugs afford greater grower control of transplanting dates, provide mechanical transplanting opportunities and allow improved water management for transplant establishment relative to fresh bare-root plants. New uses for plugs have been identified in recent years; for example, photoperiod and temperature conditioned plugs flower and fruit earlier than traditional transplants and plugs have been used for programmed greenhouse production. Tray plants have superior cold storage characteristics relative to bare-root, waiting-bed transplants. Both fresh and frozen plugs are used in a number of indoor and outdoor growing conditions and cultural systems.

The heat requirement for flower bud growth of container-grown peach trees [Prunus persica (L.) Batsch. cvs. Redhaven and Springold] in the greenhouse varied inversely and linearly with the length of the cold-storage period (SC) provided to break bud dormancy. Ethephon reduced the rest-breaking effectiveness of the 5C treatment. Buds from ethephon-treated trees grew more slowly than buds from untreated trees upon exposure to 20 to 25C, resulting in later bloom dates. The effect of ethephon on flower bud hardiness in field-grown trees of `Jerseydawn' and `Jerseyglo' was studied using exotherm analysis after deacclimation treatments. Bud deacclimation varied with reacclimating temperature (7 or 21 C), cultivar, ethephon treatment, and sampling date. All buds were more susceptible to injury in March than in January or February. Buds reacclimated more rapidly at 21C than at 7C. `Jerseyglo' reacclimated more rapidly than `Jerseydawn'. Untreated buds were less hardy and also reacclimated more rapidly than treated buds. Ethephon enhanced flower bud hardiness in three distinct ways: 1) it decreased the mean low-temperature exotherm of pistils, 2) it increased the number of buds that supercooled after exposure to reacclimating temperatures, and 3) it decreased the rate of deacclimation, especially at 21C. Ethephon prolongs flower bud dormancy by increasing the chilling requirement. The rate at which flower buds become increasingly sensitive to moderate temperatures in late winter and spring is thus reduced by ethephon. Thus, ethephon delays deacclimation during winter and delays bloom in the spring. Chemical name used: (2-chloroethyl) phosphoric acid (ethephon).

Two- to three-week-old `Sweet Charlie' strawberry (Fragaria ×ananassa Duch.) plug plants were conditioned [seven 9-hour short days without chilling (21 °C day/21 °C night) followed by seven 9-hour short days with chilling during the nyctoperiod (21 °C/12 °C night)] in September, then planted in a vertical hydroponic system for winter greenhouse production. Conditioned plugs produced significantly more fruit than did nonconditioned control plugs in January and February, but the difference was nonsignificant in March and April. Fruit yield increased linearly with height in the column (≈40 g/plant for every 30-cm increase in column height), probably because of increasing light level. When productivity is considered on an area basis (kg·m–2) and the column height effect on yield is accounted for, productivity over a 4.5-month period was 4.8 kg·m–2 for controls and 7.8 kg·m–2 for conditioned plugs. Conditioned plug plants offer the potential for increasing strawberry productivity and therefore the profitability of a winter greenhouse production system.

Strawberries (Fragaria Xananassa Duch.) classified as day-neutrals (‘Hecker’ and ‘Tristar’), Junebearers (‘Redchief’ and ‘Guardian’), and everbearers (‘Ourown’ and ‘Ozark Beauty’) were grown at a constant 21°C under 16 hour long days (LD), 9 hour short days (SD), or 9 hours with the dark period interrupted by 3 hours of low-level incandescent radiation [night interruption (NI)]. Flowering of day-neutrals was unaffected by photoperiod; Junebearer flowering was inhibited under NI and LD compared to SD; flowering of everbearers was promoted by LD compared to SD. Runner production was greatest for all types under LD, followed by NI then SD. Everbearers produced more runners than the other types. Effects of photoperiod on total, nonstructural carbohydrates varied with photoperiodic type and tissue sampled. The 3 types of strawberries were grown also under SD and NI under day/night temperature regimes of 18°/14°, 22°/18°, 26°/22°, and 30°/26°C. With number of inflorescences and runners and total dry weight per plant, significant photoperiod × temperature × type interactions were found. Results indicate that current classification of strawberry cultivars into photoperiodic type is inadequate.

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