Hardening Off and Potting On at the TC3 Farm

This week on the TC3 Farm: Hardening Off and Potting On by Hannah W.

hardening off seedlings
Seedlings Hardening Off

Now that the days have been getting warmer, if you walk up to the greenhouse on the TC3 farm, you’ll see a full table of seedlings outside, getting used to the wind and cooler temperatures. After a while, they’ll be brought back inside for the night. This process of “hardening off” helps tiny seedlings prepare for life in the farm field. As they grow, trays of seedlings are gradually moved farther from the greenhouse heater, then eventually placed on the ground, where it’s cooler still. 

seedlings ready to plantThe final steps in hardening off involve moving the seedlings outside. Even though the plastic on the greenhouse seems thin, it does offer a bit of protection from the sun, so plants also have to get used to the brighter sunlight as they transition outdoors. Seedlings may be brought outside for just a couple hours at first, then graduate to spending the whole day outdoors.

potting on tomatoes
Potting On Tomatoes

This week, the laboratory portion of class featured a tomato potting-on extravaganza. “Potting on,” refers to transferring a plant into a larger container with more soil. As the seedling grows, its roots spread out through the soil in the container, absorbing nutrients. After a while, the seedling will eventually run out of nutrients in the small area of soil it has to work with. 

Tomato seedlings also compete for light, which is another reason to pot them on. Tomatoes, as warm-weather crops, spend a long time in the greenhouse and need space to spread out their leaves. For plants like bok choi, which are cold-hardy and can be planted out in the field soon, there’s no need to move them to larger pots.

baby parsley
Baby Parsley

The limited nutrient supply in the container becomes even more important when 2 or 3 seeds sprout in one cell. In a recent post, one of my classmates explained that the number of seeds planted in each cell of a plug tray depends the germination rate. Older seeds have a smaller germination rate, so we plant a few in each cell. Still, the seeds germinate at a higher rate that predicted. If two seeds sprout per cell, that’s half as much soil for each seedling to grow in.

thinning seedlings
Thinning Seedlings

The greenhouse is a setting where space is at a premium, and competition between plants is illustrated. In this case, by removing nutrients from the soil, the second seedling has a detrimental affect on the first one. This is an example of removal interference, part of an ecological model discussed in the lecture portion of our class. For most crops, the best practice to deal with multiple seedlings per cell is thinning. The extra seedlings are pinched off and discarded, leaving more space for one to grow.

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