How should I store plant material for DNA isolation using the DNeasy Plant Kit?
FAQ ID -114

If storage is required, we recommend to freeze freshly harvested plant material immediately in liquid nitrogen and store it at –70°C for later use with the DNeasy Plant Kits. Alternatively, dry the plant material in the presence of a 10-fold excess of silica gel and seal it in a plastic bag (if the material is not dried completely after 12 hours the DNA is likely to degrade. See Mark W. Chase, Harold H. Hills; Taxon, Vol. 40, No. 2,  1991, pp. 215-220: 'Silica Gel: An Ideal Material for Field Preservation of Leaf Samples for DNA Studies').

For more information on sample storage, see also QIAGEN News article 2004 e8 'General considerations for the storage of sample material prior to DNA purification'.

To avoid overloading the DNeasy Mini Spin Column do not use more than 100 mg fresh or frozen tissue, or 20 mg dried plant tissue, for each DNA preparation.