The main reasons experimentalists store seed are;

To have it available and in a suitable condition for future use.
To have alternative batches of important species readily available, in case the germination of one batch declines.
To permit dormancy to decline.

The main questions on seed storage relevant to experimentalists are ‘how should I store seed for my experiments’? and ‘how long will it remain viable’?

The most important factor in the storage of 'orthodox' weed and crop seeds is to maintain a constant, low moisture content. This is secured by initially drying the seed thoroughly, then ensuring that it is not in contact with moist air thereafter. A constant moisture content is crucial, because if the moisture content of the seed fluctuates, metabolism accelerates in the moist periods and declines in the dry periods, with the result that the seed rapidly loses viability.

Most weed and arable crop seeds behave as orthodox seeds. A small number of weeds, often wetland species, and many trees have 'recalcitrant' seeds which are killed by drying below 10% moisture content and therefore cannot be stored for long periods. In general, recalcitrant seeds not encountered by the users of arable weed seed.

For storage of up to one year, seed moistures of 10%-14% are acceptable, but for longer term storage, 2%-5% is preferable. At these very low moisture contents, the seed is almost totally quiescent. In contrast, at 10%-14% moisture, metabolism continues slowly so that post harvest dormancy may decline and the seed can commence germination more rapidly than a seed taken directly out of storage at a very low moisture content.

For very long storage periods, seed can be dried to as low as 2% moisture, hermetically sealed to keep out moist air and stored in a deep freeze at –14C. This is a method used in many seed banks.

With a few exceptions, most experimentalists are concerned to have available a supply of viable seed of their required species at the time they need to start the experiment.

On receipt, leave the seed in its sealed plastic bags and store in a dry place out of direct sunlight. The dry seed is not strongly sensitive to temperature, but direct sunlight and extreme fluctuations of temperature should be avoided.

Stored in this manner, the seeds of virtually all species can retain viability for at least one year after delivery. Therefore you could order a full year’s requirement at one time, and the seed will be ready to use whenever it is needed.

However, the germinability of some species tends to fluctuate with time (Lamium purpureum, Sinapis arvensis and Galium aparine are typical examples). This presents problems if a key species fails to germinate half way through a series of experiments. It can therefore be advantageous to have an alternative batch of each species available. If the germination percent of one batch declines, another batch of the same species will be immediately available to substitute for it. This multiple batch strategy is a favourite among agrochemical screeners. An occasional check on the alternative batches’ percentage germination, under the same conditions as the real experiment, will enable their performance to be monitored.

If stored at 10% moisture in an even temperature and low light, virtually all orthodox seeds will retain viability for more than one year. Plants of the family Apiaceae (Umbelliferae) such as Daucus and Pastinacea have a relatively short storage life of about a year. Galium aparine also appears to become non-viable after about two years of normal storage. However most dicotyledons will remain viable for more than one year and most grasses for several years of normal storage. Avena, Festuca and Lolium can be stored for up to 5 years without loss of viability.

A pragmatic approach adopted by some organisations is to retain a batch of seed for up to a year for use in glasshouse and laboratory experiments, and to use up any quantity remaining after this period in field trials, where larger quantities of seed tend to be used.

The principle of seed storage at Herbiseed is to ensure that the customer receives seed of high viability which has been conditioned at 10%-12% moisture content so that it is ready to germinate as soon as possible after sowing. Where storage can remove post-harvest dormancy, this will have been achieved by appropriate storage

All seed produced by Herbiseed is initially dried by the sun, in polytunnels at our English farm, and in the open air in Yugoslavia and South Africa. Before it is stored, its moisture content is checked and if necessary reduced by an electrically heated air dryer.

‘Bulk seed’ of high sales volumes per year is dried to 12% moisture and stored in a storeroom which is sealed but not temperature controlled. Also seed which is known to have an extended post-harvest dormancy is stored here until its dormancy has declined, before being fully dried and removed to long-term storage.

Seed which is expected to be sold within a year (except ‘bulk seed’ and seed with an extended post harvest dormancy) is dried to 10% moisture and stored in steel filing cabinets in our ‘sales store’ which is maintained at 30% humidity, and is insulated but not directly temperature controlled. Its temperature fluctuates by less than 3C within the day, but varies from 16C-20C from winter to summer.

Seed which is to be stored for more than one year is dried to 5% moisture and stored in sealed 200 litre steel drums containing anhydrous calcium chloride. As required, it is removed to condition at 30%R.H.in the sales store.

Small quantities of seed for which sales cannot be predicted are dried to 3% moisture, double sealed and stored in a deep freeze at –14C.

In Spring and Autumn of each year, a sample of each batch of seed in all stores except the freezer is taken for germination testing. At the same time each batch is inspected for storage pests and visible signs of moisture. Germination tests are carried out in pots of soilless compost in an cool greenhouse, where soil temperatures fluctuate from 10-25C. The batches of each species which show the best germination percentage under these conditions are moved into the sales store.

Many species of seeds exhibit a post-harvest dormancy which declines spontaneously with dry storage. The length of this period differs between species and different batches of seed within a species. As far as possible, Herbiseed stores each batch until post-harvest dormancy has declined. However, the requirements of some species for specific dormancy breaking treatments such as cold stratification or heat treatment are not usually alleviated by simple dry storage. Thus it will be necessary for the experimentalist to apply these treatments before sowing the seed. (Herbiseed Brief Weed Guide No. 2 describes some of these treatments)

Weed seed for use in experiments can be stored for at least a year with confidence in simple facilities. There are advantages in storing more than one batch of important species to act as insurance against cyclic germination behaviour.

Herbiseed, New Farm, Mire Lane, West End, Twyford, RG10 0NJ, England.
Tel: +44 (0) 1189 349 464  Fax: +44 (0) 1189 241 996  e-mail:[email protected]