Answer: From your description I don't know if your greenhouse is heated or not. A smaller greenhouse can be very difficult to heat evenly. Temperature is a big influence on how quickly plants grow. Cooler temperatures will slow germination dramatically, so you might try germinating the plants in small pans indoors, then transplanting them into pots and putting them in the greenhouse if it is heated. If it is not heated, then move them to lights and then to the greenhouse as a cold frame. Sunlight is also important and some greenhouses may use supplemental lights in a cloudy year.
In general it is better to start the seeds on a time frame that allows you to have them in good active growth at transplant time. Larger is better, but at home you may not have the ideal conditions needed to start them extra early and keep them going at full steam. Keeping them in good growth means providing enough light and a large enough pot to accommodate the quickly growing transplant, so the date you start them is usually driven by the amount of room you have. As they grow they need to be potted up as needed and a good fertilization program is important, but overuse of nutrients will not help -- let the plants' condition be your guide. A plant in active growth will need more than a plant that is sitting still waiting for either more sunshine or more warmth. Overfertilizing can lead to insect and disease problems.
Every greenhouse is different and every season is different so there is no hard and firm rule about when to start plants, sometimes it is simply personal preference or when time allows! For instance, you may find that the heat loving plants are very slow while the cooler growers do fine for you. By keeping records each year you will figure out what does best in your set-up and can plan accordingly.
Finally, since your plants DO catch up once they are in the ground, maybe they are better quality than you realize. Sometimes the commercially produced plants are pushed to "bloom in the packs" and in fact are rootbound and stressed by the time they reach you even though they look so big. Their garden performance will reflect that whereas often a slightly smaller yet very healthy plant will do better in the long run -- not to mention you can also select exactly which varieties you want to grow when you start your own.
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