Yes it is a scientific fact that preventing a plant from forming seeds will increase flowering.
Certain perennials just have "so many flowers in them" and dead heading will make no difference. Flwoers like daffodils, tulips, rhododendrons fall into that category.
Perhaps the reason why her plants look so good without dead heading is is because she mostly has species that don't need it and she grows in soil with lots of compost and waters frequently.
But if you don't dead head roses they will stop blooming. If you don't deadhead shasata daisys they will stop blooming....and it IS a scientific fact...having to do with plant hormones.
I suggest Botany for Gardners for starters.
Linda C

The idea is that the plant wants to make seeds, thereby propagating itself. Once it is able to produce seeds, it decides it doesn't have to do any more work for that season. The idea behind dead-heading is stopping the plant from getting to the seed stage, thus causing it to try again. Of course, some plants are quitters and don't try again. The self proclaimed queen of dead-heading is Tracy DeSabato-Aust who wrote the seminal work "The Well Tended Perennial garden" where she goes into greta detail on the topic.

You can seal your pruning cuts with white glue (be sure it isn't the washable kind). That will keep the borers out. I didn't deadhead my butterfly bush last year and it kept blooming but the flowers got much shorter. If it had survived I would deadhead it this year. You can do an easy experiment with your roses, pick one to leave alone and another to deadhead. You should see the difference.

One of the problems with this, and many other gardening discussions, is our tendency to humanize a plant's actions by saying things like it "want's" to make seeds, etc..

Plants are chemically programmed to reproduce. When the flower has been pollinated, and begins to produce fruit, which either is or carries seed, the plant receives enymatic signals to channel nutrients and hormones designed for that process into the fruit.

If the plant is deadheaded, removing the ability to produce fruit, the plant receives enzyme signals which cause it to channel different nutrients and hormones into a renewed attempt to reproduce, by producing more blooms.

The entire process is chemically controlled, and has been carefully documented by horticulturists for quite some time. Check out the websites of the various horticulture schools if you want the down & dirty chemical processes and substances involved.

Just a note to whats already been said...

Setting seed requires a substantial amount of plant energy from rhododendrons that might otherwise be directed to formation of flower buds for the following spring. Rhododendrons not deadheaded tend to bloom fully on an every-other-year schedule (more obvious with some varieties than others)

OK, I guess it has been scientifically proven. But as to rhodies - mine bloom so extravagantly every year(you can't even see the leaves hardly) w/o deadheading that they couldn't possibly bloom any more than they already do! I have recently neglected to deadhead roses and I guess they didn't get the chemical message about already having made fruit, because they keep on making new roses!! But I'll try the suggestion about deadheading some but not others & comparing. I have so much stuff that I want to SEE a difference to motivate me to do all that work.

Actually, Donn, I don't think anyone here fails to understand that when we say a plant "wants to make seeds" we mean the same thing as when you say it is "chemically programmed to reproduce." It's just a convenient shorthand, and is most likely not a problem for most forum participants.

Sue, it's your garden, do what you want....if you believe it couldn't be any better, that's your business.
But you asked a question, and got an answer....actually several of the same answers.
If you feel your rhodos couldn't possibly bloom any better, why then please continue to do what you are doing. If your roses are blooming well enough without deadheading to please you, don't bother to prune or remove spent blooms There are no "Well tended garden" police to put you in handcuffs.
BUT...you asked a question....we gave you an answer.
Linda C

Actually, I do have a well-tended garden policeman - my husband! haha (AKA "crazed-plant-butcher-with-hedge trimmers-weedeater-lawn mower")Have to physically restrain him from cutting down the slightly yellowed bulb foliage.
His favorite excuse for plant murder - "really? I was sure it was a weed!"
Now if I could only train him to deadhead.....

I think its been asked and answered but thought I'd throw in that the years when I faithfully can deadhead, I am so much happier with the result, and Tracy di-S-Aust has made me a lot bolder with those shears. I've cut Boltonia, Heliopsis, Asters back to 12" and just crossed my fingers - it looks great. Of course that falls under pruning and pinching, but back to deadheading. I used to pick at my Shasta Daisies, now when they are mostly done I whack the whole thing down and feed it. You can get a nice rebloom instead of the few stragglers. Butterfly bush is one that really responds to deadheading by upping the flower production. If I don't nip at the Buddleias - they really slow down - its very noticable.

I've heard of getting monarda and echinacea to rebloom harder but haven't really tried seriously cutting them back to try it. I have such a deer problem I have built in flower reduction - they came through last weekend when we were gone and decimated about 100 hybrid daylilies, species lilies, and tipped the buds off of all balloon flower, rudbeckia, phlox, hosta, etc. I can only hope for rebloom.

When I read about what deer can do to a garden, I'm so glad I have a dog to patrol constantly!!! (Makes up for her occasion mulch-dig).

People over prune and prune at the wrong times. A combination of the desire for what they perceive as the 'neat' look and wanting only dwarf plants makes them whack away without understanding pruning. That can seriously impact bloom.

Have to agree with Linda. If your plants are blooming so much they couldn't possibly bloom more, why do anything to spoil the perfection?

All roses don't require deadheading, Bill Radler's for example and there are others. Perhaps your neighbor wisely chose those.

I'm always of the opinion, if it ain't broke, why fix it. What works for 1 person may not work as well for another. Deadheading does seem to work better for me, but I just don't have time to do it always. I've always deadheaded monarda, never have I gotten rebloom. Although Tracy DeSabato's book suggested cutting monarda back in May to keep it shorter, or to cut half of it back & get 2 periods of bloom. The trimmed section is supposed to bloom later. A couple of years in May I have trimed the outside edges of my monarda by half, kinda sloping it up toward the middle. For me, it all still bloomed at the same time, but makes a beautiful round mound of monarda blooms that I like much better than when it is all the same height.