Brassica Seedlings in an HMOS Greenhouse

In our last blog post we covered starting artichoke transplants and “hardening off”, the all-important period of acclimating your seedlings to the outside world. This week we’ll talk brassicas (the family that includes kale, cabbages, kohlrabi, Brussels sprouts, cauliflower and broccoli) and how to make a succession planting plan so that you’ll always have the transplants you need for an abundant, extended harvest.

Brassicas are cold-tolerant plants that should be started soon, depending on your last frost date. (Don’t know your last frost date? Find out yours here). For a last frost date of May 1st, for example, you could begin starting brassicas around March 6th. Because they are frost-tolerant, your brassica seedlings can be planted outside about two weeks before the last frost date in your area.

Starting Seeds

Brassicas germinate best between 65-75ºF, but will germinate at temperatures as low as 50ºF. You may start them on a heat mat if the ambient temperature is cool, just be sure to remove them from the heat mat and place under lights as soon as soon as they germinate. I recommend starting them in fairly large plug trays, such as the 50 cell tray included in our seed starting kit or in 4-packs, but they can also be potted up from smaller cells (like those in a 96-cell tray) once they have their first true leaves. Sow one seed per cell (to avoid wasting seed) unless your space under lights is at a premium, in which case sow two seeds per cell and then snip the weaker of the two with scissors.

Seedlings hardening off in an HMOS coldframe

Be very gentle when potting up these tiny plants, as any small nicks in the stems caused by fingernails or rough handling create an opening for the plants to become diseased. A  butter knife or dowel may be helpful to loosen seedlings from smaller plug trays, especially if they have become root bound. You can find crop-specific seed starting instructions (such as seeding depth, spacing, nutrient requirements and more) on our website here.

Here’s a tip: touch your plants! Gently brushing your hand over the leaves several times a day simulates the wind and helps them grow stronger, sturdier stems. (Just make sure yours hands are reasonably clean first). You can also set up a fan blowing towards the seedlings, which accomplishes the same thing. Keep in mind that the soil in your trays will dry out more rapidly with the increased airflow, so check on them regularly to make sure they aren’t getting dried out. Brassicas do not tolerate heat or drought well, and wilting even a few times can significantly reduce yields.

We discussed hardening off last week, and the same rules apply here. Start giving your plants some time outdoors about 2 weeks before planting, increasing the time they spend outside each day until they spend the whole day outside. Just be sure to bring them back indoors or otherwise protect them in cases of extreme weather.

Kale growing in the HMOS Trials field

Frost Tolerance

Continue to keep an eye on the weather for the first few weeks after your seedlings have been planted out, and cover with row cover or low tunnels in the event of a hard frost or hail. In general brassicas will tolerate a light frost, when temperatures dip between 28-33ºF for a few hours. Young plants are more vulnerable than mature ones, however, and must be covered to survive a hard frost, when temperatures fall below 28ºF for more than 2-3 hours. Hard frosts usually occur on spring nights with clear skies and calm conditions.

Making a Succession Planting Plan

Brassicas are a great example of a crop that you can succession plant repeatedly throughout the season to extend the harvest as long as possible. They thrive in the cool conditions of fall as well as in the spring, and can even be overwintered in many climates. Creating a succession planting plan will help ensure that you always have transplants ready just when you need them.

To make a plan, use a grid or spreadsheet to conceptualize how your garden space can be maximized over time. First determine a way to divide up your garden, such as by bed and row, and give each a name to stay organized. Write the name of each bed in the left-most column of your grid. Then use the months of the year as the headings for each column starting from left to right. To figure out how long a crop will remain in each bed, you would need to first determine the days to maturity for that variety (listed on the packet) and then add the time you’ll be harvesting from the crop to figure out how long to dedicate that space to it. But that information varies by variety, region and conditions—so let’s just stick with the broad strokes:

Short Season Crops take 30-50 days to reach maturity and are harvested over 1-3 weeks. Allow 1.5 months for these crops: Cilantro, Fennel, Salad Greens, Baby Greens, Head Lettuces and Radishes

Mid Season Crops take about 40-60 days and are harvested over 1-3 weeks. Allow 2-3 months for these crops: Basil, Beans, Beets, broccoli, Cabbage, Carrots, Cauliflower, Cucumbers, Greens (Full Sized), Kohlrabi, Okra, Peas, Potatoes, Scallions, Spinach, Summer Squash and Turnips

Long Season Crops take 55+ days to reach maturity and are harvested over a 1-3 month window. Allow 3-4 months for these crops: Artichokes, Beans, Brussels Sprouts, Celery, Chard (Full Sized), Collards, Corn, Eggplant, Kale, Melons, Onions, Peppers, Pumpkins, Tomatoes, Watermelon and Winter Squash

Lay out your crops in the grid according to how long they’ll spend in the garden. Use our Vegetable Planting Guide to determine how early to start transplanted crops. Then mark your calendar with the dates you need to start transplants in order to plant on time according to your plan. Here’s an example based on a northern garden:

And if you missed them, check out the other articles in our series: A Complete Guide to Starting Seeds: Tools, Tips and Alliums (Part 1) and A Guide to Starting Seeds: Artichokes & Acclimation (Part 2)

Happy seed starting!