It’s the first day of the best month for sowing tomato seeds.

The sun is shining in my part of the world and the urge to sow a few seeds this weekend is overwhelming!

If you have more willpower than I do, the best sowing time for tomatoes is from the middle of March to the beginning of April in the UK.

I usually start by sowing varieties that won’t become too leggy in low light. Getting seedlings through March and April in dull, short days is a challenge – always kept indoors overnight of course.

Seedlings ready for transplant
Tomato seedlings ready for transplant – just one look and I’m hooked!


Also, I like to sow bush varieties first – they won’t grow as tall as indeterminate plants so are easier to manage before planting out in their final position.

Hormone Gibberellin
This hormone (or group of hormones) causes the internode of a stem (distance between sets of leaves) to elongate and cell division to occur. Gibberellin is made in the stems, roots and young leaves.

Elongation of the internodes is a characteristic in tomato plants that isn’t helpful because it can reduce the number of trusses on a tall variety before it reaches the greenhouse roof!

Leggy Seedlings and Young Plants
If a plant doesn’t receive enough light, it will also grow tall and leggy with greater distance between leaf sets (internodes) than normal. In this case, the hormone plays it’s role to help the plant search for light above.

Too much nitrogen is also a cause of leggy plants in the early stages of growth – especially when:

  • Light levels are low
  • Temperatures are moderate
  • Moisture in the soil is high

To keep plants from becoming leggy in low light conditions, reduce temperature and especially moisture in the growing medium.

Gibberellins are often used on commercially grown dessert grapes so that the stems attached to the grapes are longer and give more room for the grapes to grow, causing them to be bigger.

The variety Shirley has short internodes as one of its natural characteristics and is therefore one of its advantages.

Anti-gibberellins are used to prevent commercial crops such as bedding plants grown in glasshouses from becoming leggy. Hormones that are used to manipulate plant growth are called Plant Growth Regulators – PGR’s.

Transpiration – Tomato Plants Perspire!
As water vapour is lost from the leaves, water is replaced through the roots.

Without this loss and replacement process, plants wouldn’t be able to absorb the nutrients they need, that are dissolved in water, and taken up through their roots.

If a plant loses moisture from its leaves faster than it can absorb moisture through its roots, the plant wilts.

If a plant absorbs moisture from its roots faster than the leaves can perspire (transpire), fruit will crack and sometimes leaves will appear bubbly/spongy as the pressure inside increases. This often happens when soil that has been allowed to become dry gets a good watering or a heavy downpour of rain!

Moisture loss and availability
If I were a tomato plant, I would probably spend a lot of my time worrying about the amount of moisture loss from my leaves and the amount of moisture available around my roots!

Keeping the right amount of moisture, or pressure, in a tomato plant is a full-time job. Get it wrong and the plant wilts or the fruit cracks or it doesn’t get enough food and that’s just for starters!

So why do we need to know this stuff … is it important for my plants?

Putting into practice
Understanding how transpiration works helps us to get the most from the circumstances in which a plant is growing. Or even change those circumstances!

Here are four conditions that effect the amount a plant transpires.

Transpiration increases as:

  • Light increases
  • Air flow increases
  • Temperature goes up
  • Air becomes more dry (less humid)

Have you noticed that when you put young plants outside for the first few times in the spring, they are more likely to wilt in the cooler conditions on the patio, than in the warmer temperatures on the windowsill!

The reason is because there is more air movement outside in the garden, so water loss increases from leaves. They may also suffer stress from the change of conditions.

One of the problems is, when plants are still young, they don’t have the root area to absorb moisture quickly enough so they may wilt in direct sunlight.

Transpiration and mineral take up
Reducing the strength of nutrient levels helps too.

Over-feeding plants on a sunny day may also cause them to wilt because higher levels of minerals in the root area, reduces the rate of water a plant can absorb because of osmosis. We’ll look at osmosis next week.

Of course the more leaves a plant has, the more water it will lose when the sun comes out and the breeze blows. A good reason for de-leafing and removing lower leaves in mature plants.

Temperature and humidity play their part too
As you might imagine, plants lose more moisture in warmer conditions. Dry air will also increase vapour loss from leaves than when it’s humid and the air is already full of moisture.

A greenhouse that isn’t well aerated and contains a lot of condensation will reduce transpiration greatly and growth will be reduced.

Also, foliar spraying in humid conditions is much less effective and is likely to increase the chance of disease.

Understanding transpiration helps us to grow plants better.

I hope the sun is shining where you are.


The most economical way, if you grow around 30 or more plants, is probably a seed tray filled with compost. A lid is also useful to retain humidity when the seedlings germinate. The humidity inside the lid helps the seedlings discard their seed husks and helps prevent stress as the seedlings acclimatise.

Another inexpensive method, if you only need a few plants, is to use a grow pot/kit which you will find in a discount store. It includes seeds and a small amount of compost – just sow in the pot and that’s it!

Jiffy pellets are also a very good way to germinate seeds and you can buy these included with a propagator. The windowsill type is very popular and useful.

It’s fun to watch the jiffy pellets expand in a tray of water!

However, the ultimate way to germinate seeds and grow seedlings is by using Root-it sponges. These come in trays and also in a standard or windowsill propagator.

The advantage sponges have over other methods is the air and moisture ratio which helps develop a very good root system quickly.

Also, like jiffy pellets, there is no need for the roots to be disturbed when transplanting. There is also no need to buy bags of seed compost if sowing into pellets and sponges.

However, you can obtain good results whatever method you use.

19 Responses

  1. Trevor
    | Reply

    Third year of planting,still a novice. First year using propagater .Planted Shirley’s a week ago,all leggy.Have read the reasons for this,but can i save them ?They are about 2″ tall.

    Regards Trevor.

    • Nick
      | Reply

      Hi Trevor,
      Two inches isn’t too leggy … transplant so that their seed leaves are just above soil level. I would also do a second sowing – it will give you something to compare them by and you’ll have spares.
      Regards, Nick

  2. Trevor Coombe
    | Reply

    Hi Nick,

    Used the plastic ‘airpots’ last year for the first time but found them to be a bugger to water!
    It all seemed to run out of the top few rows of holes!
    I think I recall you mentioning some sort of ‘wick’ last year. Can you remember where you get them?



    • Nick
      | Reply

      Hi Trevor,
      On one edge there are no holes in the top 2 inches – though it’s really not enough. They have to be filled with compost almost to the top to prevent the water running out the sides.
      The wick is just a length of capillary matting through the bottom of the pot for watering from below in trays.
      They do a good job when you get used to their peculiarities!

      • Trevor Coombe
        | Reply

        Thanks Nick,
        Yep put them together the right way up but I good idea to check I guess.
        Perhaps I should have said the water escaped through the first few rows of holes BELOW the blocked out ones! 🙂
        So, just cut strips of capillary matting will do? I’ve read on some website somewhere shoelace might work?


        • Nick
          | Reply

          Hi Trevor,
          It is a bit of a problem if you water from above with a watering can. Last season I had some of my air pots in trays watered with aqua and smart valves which worked well.
          The thing about shoelace is that it may not be able to deliver water fast enough on a warm day – I think that a good thick piece (2 or 3 inches thick) of capillary matting would be better. It’s a bit of a squeeze through the base though!
          Cheers, Nick

  3. Bob Iles
    | Reply

    That bit of sunshine must have gone to my head, I got all twitchy and had to sow some seeds. As usual this year I’ll be growing a few plants of each variety, and any spare plants will be given to friends or to our local allotment society. This year I’m planting Shirley, Ferline, Country taste, Alicante, Gardeners Delight, Sweet Baby, Matina and my giant show variety which last year gave me a tomato in excess of 21/2lbs. They are all in the propagator and lets hope for another good season. I find the debate on seed compost very interesting, but I always sow mine direct into multi purpose compost and never have any problems. Wishing a good season to all. Regards Bob.

    • Nick
      | Reply

      Hi Bob,
      This time next week and you’ll be counting seedlings!
      I find that over watering is just as likely to create leggy seedlings as over feeding. But then, if there is enough light, it doesn’t matter very much what the seedlings are growing in, as long as it’s good quality media.
      Cheers, Nick

  4. stephen clark
    | Reply

    hi Nick,
    ah the 1st of March, am also itching to start sowing but going to wait till middle of March, i hope i can avoid last years problem of BER as lost over 50% of all fruit, this year am trying out those Hoselock growbag watering systems with quadgrow system, also going to try some varieties i never tried before together with seeds recieved from other growers via this site, new varieties are st.pierre, artic plenty, oregon spring, bush beefsteak.

    • Nick
      | Reply

      Hi Steve,
      I think the Hozelock will be the answer to the BER problem. I grew ten plants last season in reservoir watered set-ups (quadgrow and auto-pot) and had just one tomato with BER.
      Your generosity last season is being repaid!

  5. John Robertson
    | Reply

    I find the best way to germinate most seeds, including tomato, is in a small Lock and Lock plastic box with a thick layer of kitchen roll placed in the bottom soaked with water and the seeds sprinkled thinly on top. Most seeds germinate within a week or so and when large enough to handle potted on into Jiffy 7’s or pots with the aid of a pencil. At lease this way you can see if germination has been successful.

    • Nick
      | Reply

      Hi John,
      I haven’t tried germination that way, but I can see the value in knowing that the result will be successful – thanks for the tip.

  6. Michael Johnson
    | Reply

    Hi-Nick and all, with regard to legginess in tomato plants, I find from various experiments that the main reason for legginess is because people transplant them far too early out of the seed sowing mix, into potting mix-thus the plants sense the extra fertilizer in the potting mix and take full advantage of this fact and grow away like crazy and stretch like anything,

    I get much better results from only transferring them in to small pots still only into seed compost mix-rather than into potting mix, it keeps the plants compact with virtually no legginess,

    From my experiences plants should only be transferred into a potting mix barely three weeks before they are planted out or transferred to their final large indoor greenhouse pots, by doing this the plants catch up and grow away into nice compact plants without any trace of legginess in most cases, if there is too much feed for them to gorge on (as tomato seedlings and plants are gross feeders) very early on in their small potting stage on the first seedling transplant, they will stretch like hell if they are transplanted into potting mix, as most people seem to do ? and also into subsequent small pots before the final potting size, far better to keep them all only on seedling mix until the last three weeks.

    • stephen clark
      | Reply

      Michael, you are spot on keep seedlings in seedling mix or seed compost because as soon as you use gen potting mix the plants become leggy fast

    • Nick
      | Reply

      Hi Michael,
      That’s a good tip for reducing the nitrogen levels when potting on – thanks for the tip.

  7. Buster
    | Reply

    Hi Nick. Sowed my 4 experimental varieties…… Amateur./Stripped Stuffer./Tamina./Bejbino. 10 days ago.
    Germination has been excellent. Used Wickes all purpose (£3.00 for 70 litre). Seed trays were well watered & drained. Wrapped with clingfilm & positioned in good light on an indoor table.
    Now comes the tricky bit!
    Thanks for the site.


    • Nick
      | Reply

      Hi Buster,
      I know Tamina very well, though I haven’t grown Stripped Stuffer or Bejbino.
      Good luck with the tricky bit!

  8. Rhys Jaggar
    | Reply

    Of the four cold tolerant varieties I sowed in February, the susceptibility to ‘legginess’ (on the basis that all 4 strains sit on the same tray on the same windowsill in the same growing medium so can be compared directly) goes:

    Sub Arctic Plenty = Black Cherry < Glacier < Maskotka.

    None are really a cause for alarm, particularly Maskotka and SAP which self-limit as bush/determinate varieties at just over 1 metre height.

    In 8cm pots it doesn't matter too much, as you can plant the plant deeper in soil when you pot up. You just strip off the cotyledons and first two stems and you are, in effect, starting again with the first remaining stems just above the top of the soil.

    As for controlling moisture, my rule of thumb based on the past 3 years is:

    Stand pots for 10 minutes in a mixture of rainwater/boiled tap water (mixed until luke warm) at a depth of one third to one quarter the height of the pot (a washing up bowl works well for this), then let the excess water drain out and then leave the plants for a week before repeating. As the plants get bigger, you can either increase the depth of water or the length of time they stand in the water (osmosis causes the water to rise up in the pot).

    For those interested in biodynamic sowing, the next window for sowing is all day on March 5th and up to 2pm on March 6th. This has the moon in Aries and when I used the same scenario in February, the seeds germinated very fast and with very high frequency. For those wishing to wait until April, the analogous dates are the 1st and 2nd of the month for standard sowings and 29th of April for late sowings.

    • Nick
      | Reply

      Hi Rhys,
      It’s good to know the potential legginess of each variety, thanks for the information.

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