Stop babying your seedlings: Hardening-off and Leaning-out

In various garden groups I’ve been a part of, there is an annual debate (and sometimes a heated-one) about when to set out tomatoes. To a lesser extent there is the same discussion about all sorts of other annuals. There is an immeasurable amount of experience, which should trump most of the rest, but there are always micro-climates, weather-gamblers, and a slough of other variables and therefore we have to sift through the endless debate and find the best time for us.

Lettuce during a heavy frost. It survived.
These lettuces in “solo-cups” could have had colder roots as well, but they survived.

But there are a couple of points I seldom hear made, that I think make all the difference: hardening off and growing lean. Sure, you’ve heard of both before, but I don’t think they are used to the best extent. Here’s a bit more in-depth explanation.

Hardening-off is almost always recommended before putting out transplants. The theory is simple, and usually has to do with temperature: The plant will be shocked going from indoor room temperatures to outdoor swings. So it’s often recommended to put them out during the day and bring them back in at night. That’s great advice, but not always the whole picture. Hardening them off should also include getting them accustomed to natural sunlight and its intensity and to soil temperature and available nutrients.

Sunflowers are typically started late. But since some dropped by birds germinate early, I thought I’d try that. They have all done great.

If you take an indoor grown tomato and subject it to near freezing temperatures, it will die. However, if you take an indoor grown tomato, put it into a cold frame during a warm (for winter) spell, then when the cold snap happens and the cold frame hits a light freeze, the plants will be “fine.”(Remember, I’m on the West Coast)

“Fine” is my term for “won’t die.” But at these temperatures, they will not grow, and perhaps they are susceptible to diseases or different pests. However, it’s just as likely that this makes them tougher, and better to withstand pests and diseases. Allowing plants to “harden-off” is literally a hardening of the plant tissue.

I successfully over-wintered celery without doing anything, but I was concerned when these new celery drooped in the heavy frost. But….
An hour later and they were good to go.

So it begs the question, is hardening off the trick, or are we starting our plants out too weak.

One thing you need to know is: fast, green growth is not your plant being strong, it is just your plant growing fast. Fast growth is a plant’s response to competition. If you make it think it’s mid summer with warm temps and bright light, it’ll grow as fast as it can above ground, since it is trying to catch-up. The easiest way to understand this is through well-proven (and easy to replicate) shade-avoidance growth patterns. Grow two identical plants with the same temperature, but one bright and one dark and the dark one will stretch its stem (behaving as though it is under a canopy.) You can do the same with temperature: give two plants the same amount of light, but keep one warmer than the other, and the warmer one will stretch. (both of these are dependant on the type of plant).

One tip I want to share with growers is to grow your plants as if it is early spring. First germinate them at whatever temperature and conditions they need. But once they have some cotyledons and a first true leaf, start to reduce the temperature (and certainly don’t give them more than a spring light schedule). It’s fine if the plants get warm in the sun, like on a windowsill, as long as they cool way down at night.

The heavy frosts outside meant it was cold in the cold-frame greenhouse. My thermometer recorded zero, but obviously there was no frost. But in here, handling zero degrees (celsius) are peppers, tomatillos, tomatoes and all types of flowers and other veggies. All were fine.

Hand in hand with this, is growing them lean. For the same reasons as above, supplying water soluble nutrients to grow them fast indoors will result in a shock when they are relying on natural and (typically) less nutrients outside. You will have created lush green growth, and the plant will not have the nutrients to feed that tissue. And even if the soil is incredibly fertile, the cooler roots will not take-up as much. So, lean-out the plants to prevent shock.

There’s another benefit to this: Fast growth results in thin cell-walls, and less supporting matter. A slow leaf will be thicker, and the waxy-coating will be thicker and these protect from sun, sucking insects and infections. A fast growth will be subjected to spores, sucking insects and greater shock by too much sun or rapid temperature changes. Furthermore, most plants have their own chemicals to ward-off pests. I’m sure your gardening experience will tell you that most sucking insect damage is on the young-tender leaves. These are the softest tissue, and seldom have the same concentration of insect dissuading chemicals.

Frosted flowers – not supposed to set out until after last frost. They were fine.

Therefore, grow your plants way leaner. One option is to use your actual soil mixed with germination media to get them started and then don’t fertilize them at all. Essentially, don’t baby them. I like to think that when I put them in the ground the conditions will be a slight improvement for them and they will grow accordingly.

When to set out your plants then becomes a decision of convenience and I’d say soil temperature. I put mine out early because I grow so many that when watering becomes a daily chore it gets tiring. But I would certainly never put a tomato in the ground until I can feel that the soil isn’t “very cold,” and I usually find tomato seedlings sprout from their last years’ location which let’s me know they are good to go.

Almost all my plants, except peppers and squash are in the ground. I’m confident putting squash in, since seeds are spouting from the compost last year, but the peppers, I will wait…in fact they may never go in.

Cabbages were fine. Prolonged cold (winter) will make them bolt. A frost like this has no affect.

There is one massive catch to this: Cold weather has an affect on plants. If you try this with brassicas or mustards and likely many biennials (like the carrot family) you will find that even planting them this calendar year, but exposing them to long freezing temperatures will make them think they have overwintered and they will bolt. But this is easy to understand and predict. Think about your kale or other plants that, if left in the ground form early flowers. In the spring they will form seeds and those will drop the ground maybe germinating in spring, or after the dry-spells of summer. So, if this is their natural tendency, then work with it. Do not subject brassicas to freezing temperatures, despite being cold crops. Seems ironic, doesn’t it? Tomatoes can be cold but cabbages cannot.

Strawflower – NOT FROST TOLERANT. But here it is with frost and is doing very well.

As always, experiment. Grow an extra couple of each plant and put them out super early or hang on to them and put them out super late, take notes and assess. Then leave me a comment, or share your findings elsewhere.

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Grow Hard

2 thoughts on “Stop babying your seedlings: Hardening-off and Leaning-out

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  1. I am new to gardening. I mostly grow ornamental. I have reading my way into edibles via apples, strawberries, Kale, Spinach, lettuce, and now Kiwi, Raspberries, blueberries etc. I am gardening in zone 7a and have the worst time with aphids. I mostly use neem oil now but have tried other chemicals including d.e. This spring I bought and released lady bugs (many of which were already dead *eye roll*. Immediately after, I found your blog. I am paranoid about what will happen if I leave the aphids for naturally occurring predators but everything you said seems to make so much sense right down to growing my plants lean so I am willing to give it a shot next growing season. I can’t wait to see the results. Thank you for your articles.

    1. I’m happy to help! Certainly, when aphids are a major problem, it’s worth looking at them as the symptom of a greater problem. But remember, they can spread disease and stunt growth. So squishing by hand and the blast of water with the hose can sometimes make all the difference when you’re waiting for natural predators to show up.

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