Growing in a Greenhouse Versus the Garden

When it comes to actually growing in a greenhouse, there are a few simple things you need to understand.
The first is that the rules of good growing outdoors are still the rules indoors. You need good soil, good fertility and good plants. Pretty simple right? Think container gardening. Growing in a greenhouse is essentially growing in containers. Here’s the data on that. Info on container gardening – things you need to know about growing plants in a greenhouse are here.
And when you need to start your own seeds of cuttings – here’s everything you need to know. All the propagation information you need is here for your growing in a greenhouse (or darn near everything).
But there are a few differences
The main difference is that you have to control the growing conditions. Too much heat and your crops suffer (so get the ventilation right) Too little heat and your crops suffer (so get the heating right).

Biggest Difference
A significant difference is humidity control. There is a massive increase in humidity inside a greenhouse and while this can be a good thing, it can also quickly lead to rots and fungal problems.
You really do require internal air movement separate from the main ventilation system. In other words, the air has to be moved around in the greenhouse at all times – when the heat is on, or when the heat is off.
So do get yourself a fan, whether it’s the blower on the furnace that never stops running or a house-fan (for a very small backyard greenhouse) and keep the air moving whenever the ventilation fans are not running. Hook up a plastic tube to the heater blower for good distribution and movement in the entire greenhouse. You can obtain these from greenhouse supply companies.
Having air moving (gently) across seedling flats and cutting tables is particularly important to stop damping off. I don’t mean whipping across so you’ll dry out the media, but a constant gentle air-flow so excess humidity is removed from around the tender seedlings or cuttings. You will find the soil dries out a bit more with this air flow.
I used to have a fan blowing across the propagation table but 2-3 feet above it so the plants weren’t blowing but the air was constantly being exchanged at seedling level.
Bottom Line
Once you get the propagation systems under control and understand how to grow a great container, growing in a greenhouse becomes one of the most pleasurable things you can do.