First a couple of questions....
Are the plants healthy? Any signs of nutrient stress? How about the health of the roots?
Assuming the plants are healthy otherwise, to speed growth up we want to think about the factors that contribute to soft vegetative growth and also consider the reasons for slow growth, maybe something can be corrected.
Fertility -
Using a fertilizer containing at least 50% ammoniacal nitrogen will promote rapid, soft vegetative growth. At this stage your crop should be ready for 200-250 ppm nitrogen but remember the ammoniacal nitrogen may lower your pH so it’s important to monitor pH closely when you make any changes in fertility.
Greenhouse conditions-
High relative humidity and moisture availability combined with good airflow in the greenhouse will increase the plants rate of metabolism and in turn encourage rapid vegetative growth. Good, warm temperatures will also promote growth. We like to see night temperatures at 68F and day temperatures between 80-85F for rapid growth. Warm days and cool nights or a positive DIF result in the most rapid stem elongation.
Spacing -
Stretch is encouraged by competition for light with neighboring plants. Keeping the plants at a pot-tight or intermediate spacing will encourage growth. Be careful with spacing though...stretch can happen very fast once the plants reach a certain degree of canopy closure and while the plants may be taller, stretched internodes don't result in the best quality plant.
You still have plenty of time to make up inches without sacrificing quality. Evaluate the conditions we discussed above, maybe you'll need to make some cultural adjustments to get more growth but you’re in a good position now before flower initiation to make adjustments if needed.
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