What reasons are there to remove dead leaves from plants? Does plant type or indoor vs. outdoor change the important of removing dead leaves?
People says it's the correct thing to do. Searching google only returns pages about how to do it and not why it's done.
I'm particularly interested in any research. I find too many anecdotes are used when it comes to questions like this.
I'm not aware of any scientific research on this subject, though there may well be some somewhere; the fact that dead or dying leaves are removed is just common sense, sort of a no brainer, really, rather than its being 'the correct thing to do'. The primary reason for removing them, particularly on housesplants, is aesthetic - let's say you have a peace lily, and that peace lily has several older, shrivelling leaves, and perhaps some dying back because of lack of water at an earlier date, so some actually dead and withered, some on the way. You might choose to wait until those which are on the way to dying are actually dead before removing them (by then, it should be possible to simply lift them off rather than cutting them), but in the meantime, you're looking at a plant with lots of dead and dying material, which renders the plant unattractive, unkempt, tatty looking and giving a general air of neglect. The dead leaves serve no purpose, and as they deteriorate, may encourage invasion by fungi and cause infection to the living parts. Dead or dying leaves might indicate a problem with the plant which might mean they should be removed sooner rather than later, and treatment given to the plant to resolve whatever issue it may have.
For outdoor plants, particularly herbaceous perennials, many people leave them to die back as they will at the end of the growing season, and just remove the dead material the following year. This might be done to provide extra protection for the roots during the winter, or simply to provide insects and other forms of life somewhere to shelter over winter. Usually outdoors, there's a leaf litter layer under shrubs, evergreen plants in particular, so those leaves have not been physically removed from the plant, but allowed to fall naturally, and cleared away at a later date. Leaves which fall from plants with fungal infections are usually cleared away as quickly as possible, to prevent overwintering spores in the leaf litter.
Dying leaves on plants outdoors during the growing season are sometimes removed for reasons other than aesthetic ones - removal of spent parts may encourage more growth/flowers, or the plant has an infestation of, say, caterpillars or sometimes a fungal infection, for which there is no chemical treatment, so any affected leaves are cut off before they're dead, provided this does not completely defoliate the plant.
Bulb plants outdoors are a different issue - daffodils, for instance, should not have their leaves cut or removed until six weeks after the flowers have finished. Photosynthetic activity via the leaves is what allows the bulb to store sufficient energy to produce a flower the following year, and that is true of many bulbs. After that time, they can safely be removed before they turn into a yellow and brown messy heap, perhaps smothering surrounding planting, or often a soggy yellow/brown tangle, if the weather has been very wet. If they're growing on their own in a large area, say beneath trees, then the leaves can be left to rot down on their own.
Remove the dying leaves so the plant will not spend its resources trying to restore that part of itself.
I removed half of the yellowing leaves from my tuberous plants such as elephant ear/ginger and laid them beside the plants. I observed that the leaves left on the plant turned completely brown and dried up faster than those removed. That doesn't make sense unless bulbs/tubers reabsorb nutrients as the leaves die.
