A few years ago, kale burst back into the culinary scene and back on our plates. Long before kale became a foodie-fad, it was a hungry gap staple. When the temperatures dip and the last lettuces wither, kale still stands green and harvest-able.
Whether curly-leafed, bumpy-leafed, plain-leafed, or leaf and spear, this domesticated leaf cabbage grows throughout the continental US. Growers in zones 8-10 should watch their high temperatures though.
High temperatures plus kale equals this bitter flavor that will send you running for a glass of water. It’s not pleasant. The questions are what is the maximum temperature for good kale flavor and how long can the temperature be above the maximum without affecting the flavor. There are no easy answers to this.
As a rule of thumb, use the average temperatures in the Planting section below for fresh market kale. This is more to protect your reputation than your palette. Many times, I’ve eaten kale grown in the last days of spring when the temperatures exceeded 90 F.
Incorporate all pre-plant fertilizer or band it 2″–3″ below the seed and 6″ away from the seed row. Note, soils high in potassium and/or phosphorus may require less than the recommended amounts of these nutrients.
Apply 60-100 lb/acre of nitrogen as follows: 1/2 pre-plant and the remaining 1/2 after thinning. The second application should be side dressed.
Apply 80–100 lbs/acre of phosphorus pre-planting in bands 2″ below the seeds.
Apply 80–100 lb/acre of potassium should be applied with the pre-planting nitrogen.
Although some varieties of kale are drought tolerant, kale tastes different when grown under drought conditions. This may make it less marketable. For best results, provide kale with consistent moisture, approximately 12-14″ total over its growing season.
Sow spring kale as soon as the soil temperature reaches 40 F. Sow fall/winter kale after the soil temperature drops below 85 F until the soil temperature drops below 45 F or until six weeks before the first hard frost, whichever comes first.
Although kale can easily withstand light and hard frosts (28 F), it cannot tolerate a severe frost (below 25 F). In zones 9 and 10, it will grow throughout the winter. In zone 8, it sometimes grows all winter long, but more often whithers sometime in January unless it’s protected by a row cover.
In most zones, row covers and low tunnels can extend kale harvest throughout much of the winter. Watch the highs though. On a sunny day, temperatures inside a low tunnel can be 5 F to 15 F higher than the high temperature. Since kale is more cold tolerant than heat tolerant, its best to set out any row covers after the first light frost but before the first hard frost.
Germination soil temperature: at least 40 F; 45–85 F for optimum germination.
Optimum growing temperature: 60-65 F monthly average. Note, kale dislikes heat and will not tolerate average temperatures above 70 F.
Note, if club disease is an issue, adjust the PH to 6.5–6.8.
Harvest: 60–90 days after planting
Due to damping off issues, a fungicide-treated seed is preferred. Alternatively, treat the field with a biofungicide according to the manufacturer’s instructions prior to sowing.
Kale Seed Spacing
Bed width: 36″–40″
Row spacing: 12″–24″
Seed spacing: 6–12 seeds/ft
Planting depth: 1/4″–1/2″
Drill seeding rate: 0.5–1.5 lb/acre
After emergence, thin seedlings to 9-12″ apart.
Control weeds by hand weeding, hoeing, and/or mechanical cultivation.
Harvesting, Storage, and Marketing
Kale leaves for fresh market bunches may be harvested weekly.
To harvest kale, start at the bottom of the stalk and work your way toward the top. Using a sharp knife, cut only the mature leaves. Leave the stalk and the upper leaves intact for future harvests. Discard any yellowing or damaged leaves.
At the Market
After harvesting, wash the kale in potable water containing chlorine or another disinfectant. Remove any dead or yellow leaves. Hydro-cool it. Spin dry it. Bunch it.
Since raw kale is a particular favorite among green smoothie aficionados, observe the same safety practices as lettuce! Wash your hands before harvesting, clean knives and crates, bleach in the cooling water, etc. See Good Agricultural Practices (GAP) & Harmonized GAP Food Safety Manual and Keeping Your Leafy Vegetables Fresh at the Farmers’ Market for more information.
Mist kale bunches at the market to keep them fresh looking. Store them in a cool place. Do not display them in full sun! As with all fresh-cut leafy herbs and greens, keep it in the shadiest part of your booth or, better yet, in the cooler.
Store at 32 F, 95% relative humidity. Shelf life is 10–14 days.
Kale may be blanched and frozen or canned.
Cover Crop Management
Kale should not be planted in the same rotation as other brassicas. Do not follow mustard or rape — both used for nematode suppression — with kale.
As kale readily crosses with nearby cabbage, cauliflower, broccoli, collards, and every other member of the Brassica oleracea group, it is one of the more difficult seeds to save for the home gardener and small farmer. The recommended isolation distance is 1.86 miles.
Collards and Kale | Department of Horticulture | Oregon State University. Retrieved June 8, 2018, from http://horticulture.oregonstate.edu/content/collards-and-kale
Commercial Production and Management of Cabbage and Leafy Greens | UGA Cooperative Extension. Retrieved May 25, 2018, from http://extension.uga.edu/publications/detail.html?number=B1181&title=Commercial%20Production%20and%20Management%20of%20Cabbage%20and%20Leafy%20Greens
Explore Cornell – Home Gardening – Vegetable Growing Guides – Growing Guide. Retrieved June 8, 2018, from http://www.gardening.cornell.edu/homegardening/scene57dc.html
Kale (Brassica oleracea var. acephala)-Damping-off (Wirestem). (2015, September 11). Pacific Northwest Pest Management Handbooks. Text,. Retrieved June 8, 2018, from https://pnwhandbooks.org/plantdisease/host-disease/kale-brassica-oleracea-var-acephala-damping-wirestem
Kale 101: Basic care and Harvesting tips – Our Green Thumb Community Garden. Retrieved from http://sites.miis.edu/ourgreenthumb/2012/03/26/kale-101-basic-care-and-harvesting-tips/
Masabni, J. Collards/Kale. Texas AgriLife Extension Service. Retrieved from https://aggie-horticulture.tamu.edu/vegetable/files/2011/10/collardskale.pdf
More growers riding the kale production bandwagon. Vegetable Growers News. Retrieved June 8, 2018, from https://vegetablegrowersnews.com/article/more-growers-riding-the-kale-production-bandwagon/
Seaman, A. (2016). 2016 Organic Production and IPM Guide for Cole Crops, 79.
Strang, J. ID-36: Vegetable Production Guide for Commercial Growers, 2018-19, 140.
Thompson, R. C. (. C. (1937). Production of kale. Washington D.C., U.S. Govt. Print. Off. Retrieved from http://archive.org/details/productionofkale143thom