Tomatoes are probably the most widely grown vegetable in the U.S. Certainly, most gardening enthusiasts who have vegetable gardens at home grow tomatoes at some stage. It is easy to grow tomatoes from tomato seeds, either planted directly in the ground or in planter boxes and pots.
Many people advocate soaking the seeds of tomatoes before planting them. While there is no hard and fast rule that says you must, pre-soaking tomato seeds do make them germinate faster. If the water is hot enough, it also kills any bacteria the seeds might have picked up, especially those you have harvested yourself.
Why should you soak tomato seeds before planting?
The main reason to soak seeds before planting them is to increase the rate of successful germination. The water softens the seeds and also speeds up germination by days.
Soaking tomato seeds in hot water is also a good way to kill bacterial plant pathogens. These may be on the surface of the seed coat or inside the tomato seeds.
It’s worth remembering that tomatoes are susceptible to all kinds of viruses. This is why farmers prefer sowing seed rather than taking stem cuttings, using tissue culture, or grafting.
As mentioned above, you don’t have to soak tomato seeds before you plant them. But, if you’ve had problems getting the seeds to germinate, it’s worth trying.
While shop-bought, commercially produced seeds are usually sterile and free from germs, those harvested from ripe tomatoes might not be. For that reason, soaking your home-harvested seeds in warm water may be a good idea.
Another reason is to see which home-harvested seeds are viable. If you pop them into half a cup of lukewarm water and then soak them briefly, viable tomato seeds will sink. You can then discard those that are floating on top.
But here’s a thought. If you’ve ever found random tomato plants appearing in your garden, do you know how they got there? One known way is if you’ve thrown overripe or even half-rotten or half-eaten tomatoes into your garden beds.
This is not a recommended method of growing tomatoes. But it happens, even though you should always dry the tomato seeds you harvest for planting. No soaking is required!
What professional research tells us
Biological scientists, S. Sabongari and B.L. Aliero did some research to determine the effects of soaking tomato seeds on germination and growth. Their research paper is titled, Effects of soaking duration on germination and seedling growth of tomato.
Their ultimate conclusion was that the process can improve crop yield and boost farmers’ income. So, there’s no doubt that it can do the same for home growers.
But you’ll see in the next section that the length of time you soak before planting tomato seeds can make a big difference.
How long should you soak tomato seeds before planting?
Advice on how long to soak tomato seeds before planting varies. Some people say you should simply soak seeds in warm water overnight.
Sabongari and Aliero’s research involved soaking and then planting tomato seeds representing three determinate, high-yield varieties:
- Roma VF – A fusarium wilt-resistant tomato commonly used for canning, tomato pastes, and puree
- UC82B – A highly disease-tolerant variety that produces firm, red fruit
- Xina – A vigorous variety that is resistant to nematodes and the Xanthomonas Campestris bacterium
They had three soaking trials for each: 12 hours, 24 hours, and 36 hours. Maximum total germination was achieved when seeds of all three varieties were soaked for 24 hours.
The highest germination rate after 24 hours was in the UC82B seeds, 98.5% of which germinated. Only 71.25% of the unsoaked seeds germinated. Roma tomatoes recorded a 98% germination rate vs 70% of unsoaked seeds. The germination rates for Xina tomato seeds were 92.5% vs 80.75%.
This shows that the optimum time for soaking tomato seeds to improve germination is 24 hours.
How to soak tomato seeds
Sabongari and Aliero were concentrating on soaking seeds for optimum germination and growth, not the elimination of bacteria.
The timing and heat of the water for soaking tomato seeds are critical when you need to eliminate bacteria. Extension educators, Jill MacKenzie and Michelle Grabowski from the University of Minnesota offer advice on preventing bacterial and viral diseases found in community and home gardens.
If people suspect that their seeds are contaminated they can treat them with bleach or hot water. MacKenzie and Grabowski say that hot water seed treatment will effectively eliminate most bacterial plant pathogens from the surface and from within tomato seeds.
They advise soaking the seeds in water heated to 100 ℉ for 10 minutes. Then transfer the seeds to water heated to 122 ℉ and soak for another 25 minutes.
Pour the seeds through cheesecloth or a thin-mesh sieve. Rinse in cool, running tap water for about 5 minutes.
They say that it is critical to meet the exact time and temperature requirements. So, you’ll need a Sous Vide, which is a cooking device designed to maintain exact temperatures in water.
Seed soaked using this hot water treatment method can be planted immediately. Alternatively, you can dry it thoroughly on a screen and store it.
Soaking tomato seeds for improved germination is a lot more simple than soaking to eliminate bacteria. Also, it stands to reason that if you use very hot water to soak the seeds, you will soak them for a shorter period of time.
The most common method home gardeners use when soaking tomato seeds, and planting them immediately, is to use warm water. Taking Sabongari and Aliero’s research study into account, it makes sense to soak them for 24 hours rather than just overnight.
Can you oversoak tomato seeds?
We’ve already said that advice on how long to soak tomato seeds varies. We have also said that Sabongari and Aliero found the most effective length of time to soak seeds before planting is 24 hours.
Soaking them for 36 hours wasn’t as effective, but it still improved the germination rate dramatically. A 36-hour soaking time was also more effective than only soaking them for 12 hours.
But in terms of growth, seeds soaked for only 12 hours had healthier growth than those soaked for 36 hours. They believe this is probably because tomato seeds absorb water when they are soaked.
This, they say, suggests that excessive soaking is harmful. Ultimately, the seeds seem to need “an optimal level of moisture rather than full saturation” to germinate and grow into a healthy seedlings.
So, yes, you can oversoak tomato seeds!
Other alternatives for soaking tomato seeds
Some people maintain that you can soak tomato seeds in cold water. There is a school of thought that says while you soak them, keep them in the refrigerator.
Another recognized method is to use coir or composted bark combined with the same amount of coarse sand, vermiculite, and/or perlite. Moisten it and put it into a clear plastic bag.
Then all you do is spread seeds around in the bag. Release the air by cutting small slits or poking little holes in the bag. This will also help to prevent mold from growing.
Store in your refrigerator or anywhere with a temperature below 40 ℉. Check the bag from time to time to see when they start germinating. If the mix gets dry, mist it with a little water.
An alternative is to sprinkle the seeds on a damp paper towel and allow them to germinate. If you do this, try to ensure that the seeds don’t touch each other.
The trick is to only spread them out on one side of the paper towel. Then you fold it in half and press the two sides together so that the seeds stick to both sides.
You then need to put the towel into a plastic Ziploc bag. But you want the air inside to escape, so poke a few holes in the bag.
Store your bags in a dark place and check the seeds now and then. If the towel feels dry, spray with a little water to moisten it.
What not to do when soaking tomato seeds
Don’t use old seeds or seeds that are of poor quality. If you do, soaking or using heat treatment will be likely to reduce seed germination.
Don’t soak your seeds for longer than 24 hours.
Only put seeds that are on a damp paper towel or in a soilless mix in the refrigerator, not those that are soaking in water.
It’s not something you absolutely have to do when planting tomato seeds, but soaking them first can speed up germination. Soaking the seeds can also ensure that you get rid of any bacteria, but only if the water is hot enough.
Research shows that the best results are achieved if you soak tomato seeds for 24 hours before you sow them. It also warns that if you soak them for too long they can absorb too much water, which can be harmful.
There aren’t any hard and fast rules, but we have provided you with guidelines that you can use. Ultimately, if you’re having trouble germinating your tomato seeds or want to get them to sprout more quickly, give soaking a try.