Seed viability

If you sow a batch of seeds and none of them germinate then it’s easy to imagine that you’ve done something wrong. Perhaps the temperature was too low (or too high), or you didn’t get the watering right. Perhaps the seeds succumbed to a pest or a disease before they even emerged from the soil. All of these are possibilities, but it’s also possible that the seed you sowed was not viable – that it was not capable of germination.

If we think back to the analogy of a seed being a survival pod, with a plant embryo lying dormant inside, then it’s easy to realise that they don’t have an unlimited lifespan. Eventually they will run out of the energy needed to maintain the life-support system, and when that happens the embryo will die. How long this process takes is very much dependent on the plant species, and it’s not a black and white thing; the percentage of viable seed in any given batch drops over time.

Seed packets often come with a Use By or Sow By date for this reason, but you can also find tables of expected seed lifespans on the internet.

It’s also possible that the seed was not viable to begin with. We can’t see what’s inside the seed, but variations in plant health and environmental conditions mean that not every seed that is created and released is alive.

Seed Banks (like the Millennium Seed Bank at Kew, Sussex or the Doomsday vault in Svalbard, Norway) go to great lengths to ensure that the seeds they collect for the future are viable and remain so. Most seeds are what scientists call orthodox, which means that they can be stored for long periods of time at low temperatures if they have been thoroughly (but not completely) dried. Stored seed batches are tested for viability at regular intervals. Recalcitrant seeds cannot be stored in this way and have to be sowed fresh. (You’re unlikely to come across recalcitrant seeds for the vegetable patch, but recalcitrance is more common in trees.)

Plants grown for seed by seed companies are kept healthy and under relatively controlled conditions, and that is one of the ways that they ensure seed quality is high. They also have to handle the seed correctly during collection, packing and storage. The UK Seed Acts govern the sale of seeds here, and require high germination rates (although it varies according to species) and quality of seeds – more so for crops and vegetables than ornamental plants.

What happens when the seed packets leave the seed company is also important. Fresh packets of seeds, stored correctly and sold and used quickly, should present few germination problems. However, once seeds are on display in garden centres or stores then they may be subject to variations in temperature that affect their viability. If you buy your seeds from the bargain bin at the end of the growing season (in preparation for next year) then the chances of being disappointed are higher.

It’s also important to note that how you store your seeds will affect their viability. If you keep your seeds in a paper bag in the greenhouse then you’re exposing them to high temperature and humidity fluctuations (not to mention mice) and their life will be shorter. They are best kept in an air-tight container indoors where the temperature is more constant. Packets need to be re-sealed as tightly as possible; if you find any silica gel sachets amongst your purchases then throw them in to help keep moisture levels down. Have some system of rotation, so that you use up older seeds before you buy new ones.

If you have problems with a batch of seeds, or you come across an old packet and want to know whether they are still viable, then you can perform a germination test.

Germination Testing

If you want to find out how viable a particular batch of seeds is then you can sow a small amount under controlled conditions. Dampen down a piece of kitchen paper, and sow your seeds onto it. Ideally you want to have a rough idea of how many seeds you’ve sown, although that may be difficult if the seeds are small.

Fold the paper over to cover the seeds, and then seal them into a plastic bag or airtight container. Make a note of the date and then leave the seeds somewhere warm (where you normally germinate your seeds, perhaps the airing cupboard or a shelf above a radiator). Check every day or two to see whether any seeds have germinated.

The seed packet will give an estimated germination time, but if you don’t have one then two or three weeks is a reasonable time to wait for most seeds – if nothing has germinated in that time then the chances are that the seeds are not viable.

Sprouting soy beans

For larger seeds like peas and beans you can try sprouting them in a jar for a germination test. Pop the seeds in a clean jar, and soak them overnight in clean water. Drain the water, rinse the seeds and leave the jar on the counter. Rinse every morning and evening until you see signs of germination (it usually only takes a few days). In the event that the seeds go mouldy before they germinate, then you have your answer – they weren’t viable.

You may find that only a percentage of the seeds germinate – which gives you an idea how viable that batch of seeds is. You may find it worthwhile to still sow those seeds, but to do so more thickly than normal, to increase the chances of enough seeds germinating. You may decide simply to replace your seeds with a fresh packet.

If the seeds are very precious, or you’re feeling thrifty, then it may be possible to save the seeds that have germinated in your test and transplant them to a growing medium so that they can grow into plants. It’s a bit fiddly, but if you’re careful it can be done.