Of cource, in most cases you may to use completely dried plant tissues for DNA isolation. I had a lot of DNA isolation from herbarium specimens and still to work with such material now and get a lot of positive results in it. You may to use different techniques to do this, not only Quiagen DNeasy. There are a lot of thecniques designed for working especially with dried plan material. But note that success in DNA isolation depend on what tissue you use and what you want to do with isolated DNA (somtimes DNA is fragmented). For my mind, the best results can be obtained by using leaves, twigs petals or some anoter tissues dried in silicagel. Usually samples stored in the same conditions by longer time will have more degreded DNA (but not obligatory). Sometimes the results also may depend on specific characteristics of plants you want to study. If you provide some more information on the aims of your investigation, I'll be able to provide more detailed information. If you want to use DNA for barconding and phylogeny by using short fragments in most cases it will be successful. But if you want to use long fragments or genomic reconstructions in most cases you will get a lot of specific problems caused by DNA degrading.

Best wishes

It is definitely possible, although as Andrii mentioned, it would be difficult to amplify long fragments. Here they assayed different protocols for DNA extraction from herbarium samples, and the best results were obtained using DNeasy. However, amplification of fragments >400 bp from this DNA was difficult:

http://download.springer.com/static/pdf/553/art%253A10.1007%252FBF02799431.pdf?auth66=1394239169_698c12d04feaea97aef73e36a3c98ccd&ext=.pdf

For a more complete description of the methods depending on the characterisitics of the plant material:

http://www.researchgate.net/profile/Lenka_Zaveska_Drabkova/publication/259700673_DNA_extraction_from_herbarium_specimens/file/60b7d52dfaa7dbcdd1.pdf

Article DNA Extraction from Herbarium Specimens

Andrii and Gustavo. Thanks so much for your detailed explanation. I would only need to identify the plant by using short fragment. So, hopefully the results will be alright. I will try to do next week.

The journal article is really useful

Of cource 100%

Definitely possible, we are also using the same kit.