After Last Chance of Frost?

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Nov 4, 2004

If you have ever grown plants from seed you will surely have heard the expression .. "Transfer outdoors after last chance of frost".

Is our timing off here ?
You may wonder why we want to write an article about "hardening off" at the beginning of winter. The reason is the multitude of catalogues which are now arriving in the mail. From summer bulbs, annual and perennial flowers seeds, to vegetable seeds. All product descriptions end with the notorious line "transfer outdoors after last chance of frost"...

The explanation for this sentence is of course very simple. If you have spent 3 months or longer growing seedlings and summer bulbs to maturity indoors, you do not want to put them outside the day before a late spring freeze, only to have your entire investment and hard work ruined. Obvious ? Yes! - So, then what exactly is our problem ?

Our problem is that the statement appears to have become automatic, almost mandatory, to the point where it has completely and utterly lost its meaning, and even started to mislead new gardeners.

Since catalogues are now flooding the mail boxes, now would be a good time to set the record straight.

What does the line mean anyway ?
Plants are typically categorized based on their susceptibility to low temperatures. Tender and half hardy plants, such as annuals, will not tolerate frost. Plants that are hardy can tolerate frost, but only after being "hardened off", e.g. gradually acclimatized to the cooler temperatures outdoors.

If a hardy plant is put out after last frost and allowed to grow the entire season this plant will return next year with no problems. Some even stay green and flower through freezing temperatures. This adjustment period outdoors is called hardening off.

However, even the hardiest of plants will die if grown indoors at 75 F and then with no adjustment are plunged out into freezing temperatures. Inexperienced growers may not realise that even hardy plants require this transition period, and so the retailers end their product descriptions with the line "transfer outdoors after last chance of frost". Ok, this sounds like a noble idea, and it would be if used when needed.

Today the phrase is unfortunately (mis-)used beyond it's intended purpose.

Not all plants are alike
Not all plants tolerate the same temperatures. The problem I have with the much repeated one-liner is its inability to differentiate between plants. If you are going to print colorful catalogues worth hundreds or thousands of dollars, then spend the money and pay a horticulturist for an hours worth of plant specific writing.