I have been tinkering with plant spacing in my garden for years. Mostly, over time, I have been widening the spaces between plants.
For bush-type green beans, I usually end up with four to six plants per square foot. Back when I planted beans closer together, so that there were more plants per square foot, the tops were such a tangle that I had a hard time finding the beans! The real problem, though, of tighter spacing is in the increased risk of disease here in the heat and humidity that is typical of the Southeast. When plants are spaced very close together, air-flow is reduced, and any stray fungal spores will find a welcome home. The closer spacing also seems to invite more pests.
A guy was talking to me once about the failure of his tomato plants to
yield much fruit. As the conversation continued, it came out that he had
used a one-foot spacing, planting four plants in four feet of row, with
a row of peppers one foot in front of them. At the time, I had no idea
that this was the standard intensive-gardening plant-spacing, and I told
him that one square foot was WAY too little space, even though my biggest problem in plant spacing has been with the tomatoes.
In my more rash youth I squished the spacing down to just a couple of feet. In those earlier gardening years, when I put them in the ground any further apart, the plants looked so small and lonely! I've gotten tougher with time and experience, and a three-foot distance between new transplants looks better to me now, but I have been pushing myself to space them out to four feet and beyond. As a result, in general, my yields have gone up.
There are many problems with closer spacing that can lead to reduced harvests. One is, as for the beans, the increased risk of acquiring diseases that are "in the wind" and the risk of spreading the disease to adjoining plants, since at the three foot spacing they are all touching each other, and the air-flow around the plants is reduced. Not too surprisingly, sick plants tend to be less productive than healthy plants.
Another is that their roots will be in each others' spaces, competing for water and nutrients. When I pull up tomato plants at the end of the season, the roots are easily two-to-three feet long. Even when the plants are four feet apart, if the plants are healthy enough to put out a good root system, there is competition in the root zone. This means that the soil needs to be augmented with more nutrients than the usual recommendation (which is geared toward research-recommended spacings of about four feet) to support the "crowded" plants, and the plants are going to need a lot of watering as the temperatures rise to summer highs.
Here in the South, anyone who plants tomatoes on a "one foot center" is going to have to put in some extra work, beyond adding extra fertilizer, to get a good harvest. To start, some serious pruning of the plants needs to be done (all the way back to a single stem), to reduce the amount of leaf-area that will be pulling water up through the plants through transpiration. Otherwise, the gardener may need to water those plants two or more times each day in the hottest part of the summer. Enough leaf-cover will need to be left to protect the fruits from sun-scald, but the pruning will help reduce the risk of disease, too. A severely pruned canopy will have better air-flow, making it less inviting to fungal and bacterial invaders, since the leaves will dry faster after rains and too-exhuberant watering.
For gardeners using an intensive planting system in a Southern summer, choosing smaller varieties might help make the spacing work better. I have planted several varieties of peppers this year, and one that seems especially suitable for planting on a one foot center is Feherezon. It has stayed in-bounds, while producing some big, tasty peppers. The bell peppers, both Napoleon Sweet and Sweet Chocolate, the Golden Greek, the jalepeno, and the Ancho peppers already have canopies that completely span the 18-24 inches I allowed for them when they were planted.
In the cooler fall and spring growing seasons, intensive spacings seem to work better. There is less heat stress, and the plants need less water as a result. The cooler weather brings reduced disease-pressure, and there seem to be fewer pests. Any gardener having trouble with intensive spacing as the summer progresses can look forward to fewer problems soon. In just about six weeks, it will be time to start planting the fall garden!