Two tier planting in greenhouses

I have seen a lot of greenhouses on this site, youtube, etc. Almost all of them have only one level planted. In prior apartments I have used 3 or 4 tier wire racks to make a simple greenhouse with plastic and have had good luck growing on all tiers, after considering the lighting needs of various veggies. I am wondering why so many people who have greenhouses do not do double or more tier planting. I know that greenhouses - even simple ones require a bit of $$. If people even did two tier planting they could build a greenhouse half the size, half the cost, energy needs, etc. I have also seen many sites that are using various kinds of pole planting to make better use of the space. Is there some reason why this is not done more often?

Partly because of soil, trying to get a decent volume of soil on a second tier is hard- you need something to support it, you have to lift new organic materials up there, you have to lift water up there, etc.

I don't like having pots with smaller volumes of soil in- because then I have to water them more often! I'm lazy though... also my greenhouse has a severe lack of light problem as it is...

I often see hanging baskets above tables of plants in commercial nursery greenhouses.

However having a second tier will obviously limit the height of the plants on the bottom tier (may matter for some types of plants) and will as mentioned compete for light.

Depending on climate and humidity, greenhouses can also have problems with insufficient air flow around the plants, leading to plant diseases. A second tier of plants would complicate any solutions to that problem. Basically the more full you stuff your greenhouse, the harder it can be to prevent mildews, molds, and rusts.

I don't think any of these problems are insurmountable, with proper planning and design and procedures. It's only a question of whether the extra space is worth the extra effort.

My friends have had good success in raising Thai basil and other tropical herbs in a four tiered system.

Peppers and eggplant thrived on the bottom while the lighter herbs were on the upper racks. The little greenhouses are managed at almost 100% humidity and allowed to get very hot.

Most plants could not survive this environment.

I think the answer relates to available space and overall cost. If you have space for one greenhouse of 20x50 feet, then using a couple of tiers to double your growing space makes sense. For a wonderful example of just how intensively functions can be stacked in a greenhouse, get a look at Will Allen's Growing Power in Milwaukee.

But, if you have space for four greenhouses that size, and you figure out the relative costs of doing multilevel in one, or just building three more, you may find that it is cost effective to build three more greenhouses. Factor in headaches with getting soil into elevated beds, watering systems for them, working in that more intensive space, possible shading issues... might make more sense to build more than to build up in one.

And down the line, if you built four, well, then by going to multilevel in each of them ....

In really hot conditions, the multilevel system works well, since there is mutual shading and humidity sharing.

This helps to control overheating.  Water drips from the upper-level into the lower level.

With some plants, it may be a problem, having this water dripping constantly but with the tropical herbs there was no mildew or other harm.