The Ultimate When to Plant Guide

February 2nd, 2010 by david laferney Leave a reply »

My Garden in April

Pay attention.  This may be the most valuable tidbit of gardening wisdom anyone ever hands you. Of course it also might not be.

When to plant – every seed packet you pick up has a little map on the back with 4 or 5 colored zones and planting dates for each zone.  Or they have cryptic advice like “whenever soil can be worked”, “after soil has thoroughly warmed”, or “after all danger of frost.”    Forget all that.  Plant when the soil is the right temperature.  Period.   Depending upon how sheltered your garden is, or if it has shade in the morning or afternoon – or if it is in a greenhouse or cold frame – those dates are just about meaningless.  But, the soil temperature will almost never lead you astray because the ground temperature changes slowly – it is slow to warm up in the spring, and slow to cool off in the fall.  Not wildly swinging with every warm or cold front.

Seed Germination time in days at different temperatures

degrees F 32 41 50 59 68 77 86 95 104
parsnips 172 57 27 20 14 15 32
onion 136 50 13 7 5 4 4 13
spinach 62.6 23 12 7 6 5 6
lettuce 49 15 7 4 3 2 3
cabbage 51 17 10 7 6 6 9
carrots 50 17 10 7 6 6 9
celery 41 16 12 7
peas 36 14 9 8 6 6
radishes 29 11 6 4 4 4 3
asparagus 52 24 14 10 11 19 28
tomatoes 43 14 8 6 6 9
parsley 29 17 14 13 12
sweet corn 21.6 12 7 4 4 3
cauliflower 19 9 6 5 5
beets 14 9 6 5 6
turnips 5 3 2 1 1 1 3
lima beans 30 17 6 7
okra 27 17 12 7 6 7
peppers 25 13 8 8 9
snap beans 16 11 8 6 6
cucumbers, summer and winter squash
13 6 4 3 5
eggplant 13 8 5
watermelon 12 5 4 3
muskmellon 8 4 3

As a general rule seeds that can germinate at a lower temperature are also more resistant to rot.

If you study this table you will begin to understand why those melons never came up – too cool/wet and they rot, too warm – they just never germinate.  You will also understand what “as soon as soil can be worked” means – a lot of things can be planted in 41 degree soil (or colder) and will just take a long time to come up unless the soil warms first like in the spring time – in which case they spring up quicker.  You can also understand this – if it will germinate in very cold soil then the plant will probably tolerate some cold spring (or even winter) weather.  So – onions, lettuce, spinach, and those which will germinate at 32 degrees can be planted any time after the weather cools down in the fall and they will basically come up when the time is right and be fine – especially if they are sheltered in a cold frame.

If you are starting seeds indoors you can see why it’s so hard to get tomatoes to come up in that sunny (but cold at night) window – those little plugs of soil do cool off quickly unlike the soil in your garden.  Those tomatoes, peppers and other warm season tropical plants will get off to a galloping start if you can consistently keep them a bit warmer – like with home made bottom heat made from rope lights.

On the other end of the chart – when it is too hot for the seeds to germinate, most plants start to stress or die from heat, especially if they aren’t kept watered.  So those cool season crops need to be planted when it is cold so that they can make a crop before it gets too hot.  Warm season crops will do great if you plant them when the soil is just barely (or almost) warm enough, and then cover them with a fabric or plastic row cover or cold frame.  Cucumbers really will come up in 3 days if you do this.

BTW, one of those digital kitchen thermometers works great for checking soil temp.   If you don’t already have a cold frame then cut the bottom off of a 2 liter coke bottle and leave it in the garden pushed into the ground like you were covering a plant with it – check the temp under that in a day or two, and you will want to get a cold frame.  Of course you can also cover your seeds with the bottom of a 2 liter bottle (or plastic milk jug) until they come up.

If you take this one thing seriously and plant your garden as early as possible, but when the soil is warm enough you’re gardening prowess will leap forward by the equivalent of 20 years of experience.  I sure wish someone had handed this to me 20 years ago!

Need planting information for something that isn’t on the chart?  Search using this customized Google search engine. Try searching for – zucchini soil temperature – for example.

Gardening Search Engine – all results are from top US Agricultural Universities.

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