If you possess old packets of seeds, or have conserved flower and vegetable seeds from plants past their prime, there is a good way to test them for viability. Put 10 seeds of the same variety in-between two wet paper towels, then place the towel-stack into a plastic grocery-type bag, leaving some air in it and lightly tying the ends closed. Leave this assembly in indirect sunlight and monitor it, spraying it lightly with water every two days if the papers have dried out. If after ten or in some cases fourteen to twenty-one days the seeds are usable, at least 6 of the 10 will have sprouted (60% germination ratio). Check on-line for the expected span of time for each variety to germinate. That will help you gauge when to give up the ghost. If fewer than six seeds among the ten do sprout within the projected time-frame, you ought to chuck them all out and purchase new ones. Each seed has its own germination speed.
From an Old Thompson & Morgan Seed Guide, with Notations from Your Editor in Parentheses:
A seed is an embryo plant and contains within itself virtually all the materials and energy to start off a new plant. To get the most from one’s seeds it is needful (necessary) to understand a little about their needs, so that just the right conditions can be given for successful growth.
One of the most usual causes of failures with seed is sowing too deeply; a seed has only enough food within itself for a limited period of growth and a tiny seed sown too deeply soon expends that energy and dies before it can reach the surface. Our seed guide therefore states the optimum depth at which each type of seed should be sown. Another common cause is watering. Seeds need a supply of moisture and air in the soil around them. Keeping the soil too wet drives out the air and the seed quickly rots, whereas insufficient water causes the tender seedling to dry out and die. We can thoroughly recommend the Polythene (plastic) bag method, which helps to overcome this problem. Watering of containers of very small seeds should always be done from below, allowing the water to creep up until the surface glistens.
Most seeds will of course only germinate between certain temperatures. Too low and the seed takes up water but cannot germinate and therefore rots, too high and growth within the seed is prevented. Fortunately most seeds are tolerant of a wide range of temperatures but it is wise to try to maintain a steady, not fluctuating temperature, at around the figure we have recommended in our guide. Once several of the seeds start to germinate the temperatures can be reduced by about 5 degrees F and ventilation and light should be given.
Some perennials and tree and shrub seeds can be very slow and erratic in germination. This may sometimes be due to seed dormancy, a condition which prevents the seed from germinating even when it is perfectly healthy and all conditions for germination are at optimum. The natural method is to sow the seeds out of doors somewhere where they will be sheltered from extremes of climate, predators, etc. and leave them until they emerge, which may be two or three seasons later. Dormancy, however, can be broken artificially .