ThermometerTomato Growing Temperatures – The High’s and Low’s of Tomato Growing!

We all know that tomato plants need to be kept warm to grow successfully and that they like lots of light.

However, keeping them at an ideal temperature is next to impossible unless you have a professional set-up – so let’s talk about some realities!

Ideal Temperatures

  • Germination is best around 21C.
  • Keep seedlings and young plants around 18C (if possible!).
  • Outside – above 15C and best not below 10C at night – difficult to achieve if growing outdoors!
  • When tomato plants reach around 35C they stop growing and the same is true when temperatures are much below 10C depending on the variety.
  • Humidity combined with low temperatures also has an influence on growth where condensation is high – see transpiration.

Germination will take about five days if the temperature is kept at 21C but if temperatures vary widely, it will usually take longer.

Seedlings are grown in the spring (of course!) but the problem for many regions is that light levels are low and days are short in spring-time.

Temperatures kept too high
This means that if temperatures are kept artificially high with low light, seedlings will become leggy.
Temperatures too low
If temperatures drop too low and seedlings are sat in cold, water soaked soil, “damping off” where they wither and die!, is a possibility along with poor growth and fungal disease.

I’m painting such a gloomy picture – again!

How to cope with high’s and low’s
If temps are too high or too low it is best to under water rather than over water seedlings. This will help stop them from becoming leggy and reduce the risk of disease.

I once left two young plants out on the patio at night and the temperatures dropped below freezing! Fortunately the plants, an Alicante and Moneymaker completely recovered and produced one of the best harvests I’ve ever had from these varieties.

Healthy plants can cope with adverse conditions – on the odd occasion, but I would never try this on purpose!
Also, it was a dry night so the plants didn’t have to cope with wet leaves – the combination of cold temperatures and wet leaves are the worse possible conditions for tomato plants.

13 Responses

  1. Caroline
    | Reply

    Hi Nick,
    I have suffered a few years of tomato blight on my allotment now and have resolved to grow only blight resistant varieties. I am trying almost every variety I could find. I just wondered if you or others could recommend particular varieties?

  2. Jane McAllisterist
    | Reply

    I leave tomato plants out during day but bring them in at night if temperature likely to fall .seem fine but is this ok

    • Nick
      | Reply

      Hi Jane, I don’t know where you are in the world, but if you are in the UK or USA, you can leave plants out overnight now.

  3. Dave
    | Reply

    My sungolds look the weariest of all. Nigh on three quid a pack were they for just eight seeds yet they hardly show themselves for all of nine days in Levington seed and cutting compost. Since they lie beside cucumber seedlings which are eager to reach the sun and will need transplanting , they are in peril of disturbance , for soon I must prick those cucumbers out.

    • Angela
      | Reply

      I’ve had the same trouble in the past and now sow all my seeds in the small individual cells. Because they are ‘plastic’ you can cut them to length and width so they fit easily into whatever size propogator / tray you use and because they can also be cut into individual cells you can remove those that are germinating faster than others. The ones I use are about 1″ across, but they do do an assortment of sizes. I’ve found this method invaluable as I only need to grow enough for myself and it enables me to do small quantities – I label them with an oange stick and make a small ‘flag’ from a sticky label so I know what’s where!!

      • Dave
        | Reply

        I had determin-ned afore time dear lady, whereupon thy kind and proven advice is given, to deploy a method that scholars of the tomato growing art would perhaps think not meet or proper.

        See now, how it is that some of those I prick out appear with all of a tap root but some with no tap root at all, so thinkest me now to avoid such disturbances as would sever the root from my poor seedlings. I shall now sow each seed into the smallest of pots, and thereafter when seedlings are grown, I shall transfer them to pots yet larger, before they are fixed to their final place in pots yet larger than those in which they had rested before , that they shall yield me much in abundance the fruits which providence ordained should bear me before Adam’s curse. I ‘ope I shall be spared the blight, or they’ll be fed to an ’orse. Amen.

  4. Dave
    | Reply

    As a novice I ponder upon the conditions required to raise tomato plants. I have sowed several varieties all which are sprung, yet I do wonder and lament that my sweet millions , leggy though they are for lack of light and a good soaking, have yet to shed their husks. Those of other sorts that seem more hardy had o’er night appeared with husks discarded. Me thinks how I should tend these tender seedlings, to rid them of their husks, or by so doing I should do them more harm than good. Perhaps they will rid their husks by themselves.

    • Nick
      | Reply

      Hi Dave,
      I like your turn of phrase!
      I used to work as a musician at the RSC in Stratford upon Avon, so I’m quite at home with your comment!
      Try to keep the seed husks soft by misting them – it should help the seedlings discard them more easily.
      Best wishes,

  5. Richard Conkey
    | Reply

    This is excellent information and how to cope with the ‘highs’ and ‘lows’ is a great aid for an amateur ‘first time’ tomato grower.
    Many thanks.

    • Nick
      | Reply

      Thanks Richard, I appreciate your comments!

  6. Roy
    | Reply

    Hi Nick
    Many thanks for all your exellent expert advice regarding growing tomatoes from seed.
    I have grown different varieties of tomato from seed outside and in a greenhouse.
    I have been very successful over the years.but there is always something new to learn
    And I am always happy to listen to people like yourself giving advice from your own

    • Nick
      | Reply

      Hi Roy,
      Good to hear from you and thanks for your comment!

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