A dry Lulav:
A Lulav which majority of its leaves have withered, or majority of its spine has withered, to the point it has turned whitish should not be used unless absolutely no other Lulav is available. If it has dried to the point it can be broken by touching it with ones nail it is Pasul according to all opinions.
What is the law if the Tiyomes has dried?
Seemingly according to Admur the Lulav remains Kosher and Mehudar, however other Poskim rule hat one should not use a Lulav with dry Tiyomes. Practically a sunburned Tiyomes is not considered dry even according to those which are stringent.
What is the law if the dryness of the Lulav is only noticeable after close examination?
It is valid, as the invalidation of Hadar is only applicable if viewable at first sight.
If the majority of the leaves or spine of the Lulav have withered the Lulav is invalid. Admur records a dispute regarding the definition of dry: The first opinion rules that only if a nail is able to break the leaf due to its dryness is it considered dry and is invalid. The second opinion rules that even if it has not dried to this point but has lost its green color and become whitened it is invalid. Admur rules that the main opinion follows the second opinion and although Bedieved or if no other Lulav is available one may be lenient like the first opinion. [ibid]
 The reason: As it is no longer considered Hadar. [ibid]
 Raavad brought in M”B 645/22
 Piskeiy Teshuvos 645/7
 Piskeiy Teshuvos 645/9