What is the very closest together I can plant rhododendrons?

I have purchased three rhododendrons and would like to form a thicket with them. They are Henry’s Red, which should have a maximum spread of 5 feet in my zone (4B). What is the very closest I can plant them together without compromising the wellness of the plants?
Thank you!

 

One thing that should always be borne in mind is that while the Rhodo books give an average size at 10 years of age, most Rhodos don't stop growing when they meet a certain size - given good growing conditions they just keep on growing. That means that when your plants are 20 years old there is a good chance that without pruning, they could be 10 feet wide!

A great example of this is R. williamsianum, which is a lovely plant with pink flowers and small rounded leaves, usually sold in a 1 gallon pot or smaller. Down at the RSF they have hedges of this species that are chest high and or 6 feet across!

Your answer is to space so they have room for 5 years or so growth (so it doesn't look sparse) and postpone having to thin or move with some periodic pruning.

The American Rhododendron Society (ARS) has an excellent plant resource that indicates an average of a 4 foot size on your plant in 10 years

 

A close grouping of multiple specimens of one kind of shrub or perennial to form a flowing shape is called a drift. Drifts are fundamental building blocks of fully realized informal planting designs. Spacing for mature development has little to do with using plants to make drifts - the drift is the individual unit in this case rather than each specimen contained within the drift. So the spacing between each plant is much closer, with quite close spacing not being a problem for rhododendrons - which may often form thickets in nature anyway.

And since rhododendrons are bothered, even eventually killed by heating up of the root zone starting off with them near enough to one another to produce their own at least partly shaded area has a cultural basis as well.