How To Work With Vegetable Seedlings in Southern Ontario in April and May

Please feel free to use this information when planning and planting your garden. Also feel free to share with your family, friends and neighbours! We simply ask that you credit Adam for his work in writing this.

  1. Cold Hardiness


Many people cling to an old gardening cliche, “don’t plant before May 24.” This is relevant only for some kinds of crops, but not all. Plants which cannot handle any frost or temperatures much below 5 degrees celsius include the nightshade family (tomatoes, peppers, eggplant, tomatillos, ground cherries), the cucurbit family (melons, cucumbers, and various summer squash {including zucchini} and winter squash {including pumpkins}), beans, basil, celery/celeriac, and some flowers such as cosmos. These plants should not go out in the garden until at least mid-May, when the 7- to 14-day weather forecasts show nighttime temperatures above say ~8 degrees, and even then these plants should be “hardened off” before transplanting (see below).

Lots of common vegetables and herbs, including ones we grow in the field and sell seedlings of, are able to handle temperatures approaching or passing freezing, or lite to moderate frosts on nights between 0-5 degrees celsius - though there is a caveat here. Seedlings of these cold-hardy plants need to be “hardened off,” that is, incrementally introduced to outdoor conditions. When we start seedlings in our greenhouse, they start their life around 15 degrees celsius, never get much below 12, and can spend parts of days well about 20, even when it is chilly outside. If you were to take a plant from these conditions and then suddenly plant it outside with no preparation, it might get easily damaged by cold, and/or might “go to seed” due to the stress (which means you don’t get the leafy/head/root crop you were hoping for, I will do a future post about seed saving). Hardening a seedling involves putting them outside during the day, so they can get exposed to colder-than-indoor temperatures in doses, as well as get used to harsh direct sunlight (vs the softened sunlight you get in a greenhouse or on a window-sill), and buffeting winds; and then bringing those seedlings back inside a safe warm indoor space at night, or even during a colder day if there is a cold snap. After a few days (say 5-10) of this kind of cold exposure, these cold-hardy plants can then be transplanted in the garden.

Plants which can handle colder temperatures include peas, many root vegetables (such carrots, beets, turnips, radishes, onions) and many greens including: lettuce (and it’s cousins frisee, escarole, and chicory/dandelion); spinach; chard; and basically the whole brassica family (kale and it’s siblings collards, cabbage, broccoli, cauliflower, brussels sprouts, and kohlrabi; and their cousins arugula, mustard greens, bok choi, mizuna, etc); and many herbs can handle cooler temperatures too including annuals herbs in the carrot family (dill, cilantro, parsley, and fennel) and perennial herbs like sage, oregano, mint, lemon balm, chives, thyme, sorrel, lovage - just naming the seedlings we sell.

Any of these cold-hardy crops that we are selling seedlings as of the moment I am writing this (May 3, 2020) are hardened off and ready to plant. 

Whereas any of the warm-season crops listed in the first paragraph (tomatoes, beans, celery, basil etc)  have not yet been hardened off. That means that if you have picked-up or taken delivery of these plants already: put them outside during the day (when above ~5 degrees celsius) and take them in at night for the next week or so; thereafter keep an eye on the weather forecasts, and consider keeping them outside some warmer nights. Depending on the weather, you could hazard transplanting them into your garden beds/pots as early as May 10-15, or you might want to wait until closer to the traditional dates of “May Two-Four.”

If you do want to plant those warm-season crops earlier, you can always do so, and just be ready to cover them up at night with any kind of plastic or fabric. People have been known to protect young tomatoes/cucumbers/etc with tiny greenhouses out of half a plastic 2L bottle, build more elaborate “cold frames,” or simply cover up with a blanket or tarp for colder nights. In the field, we use translucent fabric known “row cover” or sometimes known as “remay” to cover whole 100’ beds of young plants to produce a minor greenhouse-like effect - though we use that primarily for earlier plantings of cold-hardy crops, we don’t typically rely on it for early plantings on warm-season crops. 


2. Transplanting Spacing


We’ve been asked before “how far apart should I plant these.” It’s a great question, because the distance between plants in the garden impacts how big they can get and how healthy they can be. 


This write-up is assuming you have healthy soil, direct sunlight, and adequate water - because any lack of these conditions will limit how big plants can get. Also a note on rows/beds. This advice is assuming you are growing in some kind of beds which, regardless of length, are 2’-4’ wide with 1’-3’ walkways between.


To use lettuce for an example: you could plant lettuce as close together as 2-4”, and you will get some nice tender baby lettuce leaves, but you won’t get a big head of lettuce. For a head you would want to plant them at least 10” apart from each other, you might even want to consider 12 or 14” apart. In the field we think about “do we want a baby salad mix, or do we want a head.” We also consider “how many seedlings do we have, for how big of a bed.” The spacing I mention here applies for pretty much any green related to lettuce, and for most asian and mustard greens. In the case of bok choi, I’ve seen people go as far as 24” apart to get rather large heads, though we typically go with 10-12” spacing. 

[It’s worth mentioning here that in our farming context, we only expect one harvest when we are cutting a head, vs two or more harvests when we are cutting baby leaves. In either case, we expect the plant roots/stems to survive after the leaf/head harvest, and to “go to seed” afterwards as a stress response. As  farmers, we are then done with those plants, and clear those beds for another crop (unless we want to save seeds). Whereas for you home gardeners, you could always leave the plants roots/stems to regrow, and you might be pleasantly surprised with more nice leafy growth; plus in the case of the brassica family, you can eat their flowers too.]

If the seedlings we delivered to you have more than one plant per cell (which happens in part because we would rather over-full cells vs empty cells), you need to decide whether you want to separate the seedlings or plant them together. To separate, just be gentle, but don’t worry too much if you rip a small percentage of roots. If planting together (say, 3 lettuce plants in one cell), it can work to give that cluster extra space, and each plant may get close to full size [a good example of this is onions, which can be planted as clusters 8” apart rather than single plants 4” apart]. 

Kale and chard are a bit different than the other greens, because they are more reliable for “cut and come again,” you can harvest leaves off of the same plants all season long, and they thrive with more space. To get full-sized leaves (like what you get when you buy  a “bunch” sales unit) we like to plant our kale 20” apart, and our chard 15” apart. Unless we want little baby leaves for salad mix, then we go with 3-6” apart. 

Kale’s siblings cabbage, broccoli, cauliflower, and brussel sprouts all need 18”+ to get a full-size crop. We usually stick to 20-24” inches apart, but some people do as much as 30”. Different story with their sibling kohlrabi, who doesn’t get as big, and can thrive just fine around say 12-16” apart (we read and tried 8-10” once, but weren’t happy with the result).

Green & bulb onions do well 4-6” apart, or clusters further apart. Leeks can handle that spacing, but in our experience they might prefer more like 8” as singles and 12” apart as clusters.

Celery and celeriac we default with around 12-14” apart, though the range used by other gardeners or farmers might be 8”-18”. 

As for herbs, some do well closer together, some further apart. Dill and cilantro we usually direct-seed, averaging maybe 1” per seed in the row. But for an early first crop we like to transplant seedlings, usually as a cluster of a few plants per cell (like what you get in a 4-pack of seedlings). These clusters we like to plant 4-6” apart. Similar spacing for parsley - 6” has always worked well for us. Different story for basil: in our experience, 10-12” apart works okay for smaller/shorter plants, for earlier harvests (or a later succession planting). But if you want to grow large waist-height basil bushes, spacing more like 18”-24” apart is the way to go. Fennel falls somewhere between parsley and basil, we like 12” apart for a modest-sized bulb. And for all the perennial herbs, 12” apart is a good default, but there’s nothing wrong with closer or further apart. As a perennial plant, they will continue to live for years, and slowly spread out (or quickly in the case of mint), so you should plan ahead when thinking about how far to plant them.

And now onto the fruiting crops. 

The outlier here is beans, which we only direct seed out in the field, but offer as a seedling for our gardening customers. When seeding in the field, we place seeds about 1-2” apart, similar for peas. But you’ll notice what you get as a seedling is a cluster of a few plants in a pot. We recommend - don’t separate the plants, rather, transplant the clusters whole, around ~6” apart from each-other, to mimic the equivalent of plants 1-2” apart. 

Almost all the other fruiting crops need 24”+ apart from each-other for good growth, depending on varieties, and if/how you trellis or prune them. You need to consider that if planted too close together and not pruned, you could end up with plants which have stunted growth (and thus don’t give a good yield), and that a jungle of foliage with minimal airflow is perfect for undesirable fungal diseases like blight and mildew.

Tomatoes are a great example. Most varieties that we sell are “indeterminate” or “vine” tomatoes whose leaders can grow more than 10’ long/tall, and which can grow dozens of secondary “suckers” which can make them bush out multiple feet in every direction (and which can yield more fruit, but delayed behind the first main stem, and generally leading to smaller fruits and and delayed ripening over the whole plant) . It is possible to plant said un-pruned tomatoes 5’ or more apart, and never prune or trellis them, letting them sprawl out over the ground - but you run the risk of fruits rotting when touching the ground, and you will have a hard time harvesting. More common is to trellis the plant on anything (bamboo, wood stakes, tomato cages, old bare christmas trees from behind your shed…), to keep most of the growth off of the ground; and even then, whether you want to prune suckers or not influences how far apart you should plant. So at one extreme, no pruning, you might want to consider 5’+ apart. At the other extreme, pruning every single sucker and having a perfect trellis, some commercial growers plant their tomatoes as close as 10” apart within their rows (but then still 3’-8’ between rows). Different story for bush tomatoes like the roma tomatoes we sell; they won’t spread out so far, so planting them 2’-3’ apart is ideal. 

I would place cucumbers, melons, and winter squashes in the same category as unpruned tomatoes. If you have some kind of trellis, consider planting them around 3’ apart (per cluster of seedlings in a 4” pot). If no trellis, consider further apart (especially for the winter squash and melons). Most summer squash varieties grow more like a bush rather than a vine, so around 3’ apart (with no trellis) is great.

As for tomatoes’ cousins: peppers, eggplant, and ground cherries typically do not grow as large as vine tomatoes, and are generally happy around 2’-3’ apart (and possibly grown as 2 rows in a bed, vs 1 row for tomatoes); and tomatillos, we’ve seen get as large as vine tomatoes and thus like to plant them around 3’ apart if given some support like a tomato cage. 

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