Winter Musings

Winter cover crop B.R.
 (before rabbits)

Cover crop A.R.
(after rabbits)

As I look at my winter cover crop planting clipped low by rabbit(s), I’m waxing philosophical about gardening and working with nature. It was planted in late September and is well established with the warm fall and winter to date.

While much of the top growth is lost, I know there is substantial root growth that also will provide organic matter for soil improvement. 

It will grow out again early before I turn it under in spring so I’m not too worried about gaining benefits from this winter rye/Austrian winter pea mixed planting. There is the perfect and then what results once the real world has its say.

Speaking of the real world, I heard lots of complaints about poor tomato yield last season. Me too. While we didn’t have any 100 degree weather, there was enough high eighties and ninety degree weather to cause poor pollination and blossom drop. 

My plants finally achieved blossom set in mid-summer and particularly the small-fruited varieties had time to produce a late-summer crop.

It’s good that every year is different and hope springs eternal with gardeners including me. I already have purchased tomato seed to start this season’s transplants come spring.

 Photo Credit: Both photos Carl Wilson 

Turning Under the Winter Ryegrass Cover Crop

As I was strolling the grounds of Denver Botanic Gardens this week enjoying the spring bulb bloom, I saw gardeners turning under a plot of winter ryegrass. This reminded me that cover crops aren't only used in vegetable growing. They can be used anywhere soil improvement is needed including for annual ornamental plantings at DBG.

Growing cover crops has enjoyed a recent rise in popularity in farming as concerns about soil loss have increased. The resulting increase in organic matter from growing cover crops helps in many ways. These include an improvement in soil structure and resistance to erosion, better water penetration and holding, increased soil biological activity, better plant nutrient holding and more. Home and market vegetable gardeners should seriously consider the benefits of cover crops in their efforts.

When growing winter rye for the first time one important question is when to turn it under. Consider this question from two standpoints: 1)how to get the most benefit and least drawbacks in the burial operation, and 2) when you want to plant your vegetable crop.

"Growback" (resprouting)
after turning under

Winter or cereal rye is best turned under when it is between 12 and 18 inches tall and relatively succulent. If turned under when short and still in a vegetative growth stage, there is a pronounced tendency to "grow back" meaning more work in burying plants a second time.

If left to grow until taller than 18 inches, rye enters the reproduction (flowering) stage and tends to have a high carbon to nitrogen ratio due to the high cellulose and lignin content that develops to stiffen stems. This means the plant parts you bury in the soil can be slow to decompose. Note that not only plant height but more so day length promotes flowering. Winter rye flowers when days reach 14 hours in spring.

Chop leaves into small chunks that are easy for soil microbes to attack and digest. In my small home garden plots I use a hedge shears cutting off 2 to 3 inch lengths from the top of the plant and working my way down to the soil line. You can also use a string trimmer to chop plants before incorporation into the soil. Chopping tends to minimize "grow back" because the food supply is cut off from the roots.

The second consideration in when to turn under a cover crop is when you want to plant. Allow a minimum of a month for the leaves and roots to break down before seeding or transplanting. This allows soil nitrogen availability to stabilize after being temporarily tied up by the soil microbes

chewing through the freshly buried rye plants. Once broken down, soil microbes release the nitrogen they tied up making it again available to plants.

Now is generally the time to turn under your winter cover crop if you are planning to plant warm season vegetables in late May or early June.

Photo credit: All photos credit Carl Wilson.

Cover crops for a green winter vegetable garden

Winter rye/Austrian winter pea

Fall planted cover crops make me feel good for two reasons. First I know that after they grow over the winter and I turn them under in the spring, I will have improved my vegetable garden soil. Second is the novelty of looking out my window in the depths of winter and seeing green plants in my vegetable garden.

Plant cover crops in fall from mid-September to mid-October on the Front Range of Colorado. With this year's warm fall weather you probably could have planted through the end of October. The season before we had subzero weather the second week in November so late plantings likely would not have survived. Plants require at least a month of moderate fall temperatures to establish before winter cold slows growth and soil temperatures drop below 40 degrees F.

Winter rye/Austrian winter pea or winter rye/hairy vetch mixtures work well for the Front Range. Many gardeners plant winter (cereal) rye. Grass (the winter rye) alone works well for increasing soil organic matter but if you want the advantage of the nitrogen adding abilities of legumes, add winter pea or hairy vetch in a mixture. Hairy vetch is hardier than winter pea and winter rye is very hardy. Plant at 4 to 6 ounces per 100 square feet except a lower rate of 2 to 3 ounces for vetch.

Water at planting and perhaps once or twice more to establish. In general winter snows will provide enough moisture for plants although you could always winter water in extended warm, dry winter weather if you feel you need to.

In spring spade or till the crop under the soil burying both tops and roots. Keep in mind that after turning under your cover crop you should wait a month for plants to break down before planting vegetable seed or transplants. If you need the garden space to start spring crops and don't have a month to wait, harvest cover crop plants and coarsely chop to decompose in the compost before adding them back to the garden soil between your spring and summer crops.

Cover crops should be a routine part of maintaining a healthy and productive garden soil. Online sources for cover crop seed include Johnny's Selected Seeds and Urban Farmer Seeds.

Photo credit: All photos credit Carl Wilson.

Greens crop replacement

Buckwheat seedlings following
harvest of spring lettuce crop

The onset of ninety degree F weather along the Front Range means those cool season greens that have lasted so long this year due to a cool May will soon be gone. The heat decreases quality (bitterness), long days induce bolting, and the crops days to harvest may have just ticked by.

What to do now? In late June to mid-July you can begin planting mid-season crops for late summer or fall harvest. Some crops tolerate heat well such as Swiss chard, bush beans and New Zealand spinach. Collards can be planted up to 3 months before frost by direct seeding. Root crops that mature in 50 days such as beets and carrots are also good bets.

With other vegetables it is best to chose heat tolerant varieties. With lettuce the Cos (Romaine) types as well as others noted for heat tolerance (such as 'Muir' cultivar Batavian type lettuce) can work. Note that lettuce seed has a natural thermal dormancy and seed may not germinate well at high temperatures. Pre-germinate seed, plant in cooler weather and use irrigation to cool soils to obtain germination.

Likewise with spinach look for heat tolerance such as in the Asian arrowhead types; one example is 'Flamingo' cultivar.

Keep in mind crop rotation when second cropping, rotating to a crop from a different plant family. In addition to edible crops, remember you can also utilize a summer green manure (soil building) cover crop such as buckwheat, Fagopyrum esculentum. Turn under in 30 to 40 days as it starts to flower to increase soil organic matter.

Photo credit: Buckwheat seedlings - Carl Wilson

Rye cover crop thoughts

Winter rye also called cereal rye is growing well in these warm days of spring. There are advantages to turning under now while it's still succulent. There is less nitrogen tie-up than waiting until later when it develops stout stems. Remember to allow at least a month after turning under before planting.

Winter rye (not Italian ryegrass which is different) is one of the hardiest cereals and is also drought tolerant. It can be planted early or late in fall, either by itself or with a legume such as hairy vetch or Austrian winter pea for adding nitrogen.

By itself it will scavenge soil nitrogen from last season and keep it from leaching over the winter. It grows well on sandy soils and is good for preventing wind erosion. Rye produces a lot of biomass adding to soil organic matter.

You can also leave a row of rye to shield early planted vegetables from spring winds. Peas, lettuce and other cold soil tolerant crops can now be seeded as soils should be well over 40 degrees F along most of the Front Range.

Photo credit: Winter rye closeup and in bed - Carl Wilson

Grow on! - Hairy vetch

It's nice to see something green in the vegetable plots in winter and hairy vetch, Vicia villosa, performs well as a cover crop. It is is a high N producer, vigorous grower, very drought tolerant, prospers in low fertility and a wide range of soil conditions, and is winter hardy to zone 4 with snow cover.

One of the best things about fall planted hairy vetch is that it provides improvements in crop yields greater than the nitrogen added by it's symbiotic nitrogen-fixing bacteria alone. This may be due to root penetration that leads to better vegetable root development, physical soil improvement that allows a better air-moisture balance for crop roots in soil, improved soil biological activity or other reasons.

Note that hairy vetch doesn't build up long-term soil organic matter. It is a essentially a succulent plant that has a relatively low carbon to nitrogen ratio in the range of 8:1 to 15:1. Most benefits are realized by the first vegetable crop that follows in spring. For this reason it is often interplanted with winter rye, Secale cereale, that has a higher C:N ratio (ranging from 25:1 to 55:1) and thus breaks down more slowly. The combination planting results in some of the best of both worlds - N contribution plus short and longer term organic matter increase.

Hairy vetch growth is almost non-existent in mid-winter but will increase with lengthening days. Mow and turn it under at 25% bloom when N contribution will be maximized.

Note that freezing temperatures under 5 degree F can cause some winter kill if there is no snow cover. Temperatures in this range occurred along some parts of the Front Range December 5 and 6 and some damage may be seen. This is another reason to plant with hardy winter rye so an adequate spring yield of biomass can be gained. Next post will highlight workhorse winter rye as a cover crop.

Photo credit: Hairy vetch closeup - Carl Wilson

Season Endings and Beginnings

Vegetable Endings
With night temperatures predicted to drop into the thirties F this coming weekend, it's time to harvest what remains of tomatoes, peppers, eggplants and many other fruiting vegetables. Thirty degree temperatures are flavor killers for tomatoes anyway so using ripe fruit and ripening mature green or pink breaker fruit indoors is preferable.

Cover Crop Beginnings
Clearing plant refuse off the growing area now has another advantage. Mid-October is a good time to plant winter cover crops for soil improvement. Maintaining desirable levels of both organic matter and nitrogen in your soil is important for growing vegetables. Nonlegume cover crops help with organic matter and legumes can add both.

One common cover crop planted in our area is winter rye but there is a lot of confusion about what that is. Winter rye is cereal rye, Secale cereale, the same rye used for grain. Annual or Italian ryegrass, Lolium multiflorum, can be used as a cover crop in certain circumstances but winterkills in cold, dry Front Range conditions. Annual rye should be sown in early fall, so it's now late to plant.

Winter rye on the other hand is one of the hardiest of cereals and can be seeded later in fall. Growth is rapid in cool fall weather and its quick-growing, fibrous roots hold soil and leftover fertilizer well. It is also good at suppressing weeds.

Rye can also be sown with legumes in fall. Hairy vetch, Vicia villosa, is one of the most popular. It's winter hardy to zone 4 and can work in zone 3 with snow cover. It grows slowly but root growth continues over winter and vine growth quickens with arrival of spring. It tolerates poor soils (including sandy ones) and delivers a heavy contribution of nitrogen compliments of it's symbiotic nitrogen fixing bacteria.

A type of field pea (Pisum sativum subsp. arvense), Austrian winter pea, can also be used as a winter legume. It is not as cold hardy as hairy vetch and should be seeded in early fall. Plant it with a winter grain such as rye to protect pea roots and maximize winter survival. It prospers best with some winter moisture.

A rule of thumb is to plan for at least a month in spring after turning under cover crops to allow them to decompose before planting vegetables.

Photo credit: Tomato harvest and Rye/Hairy vetch seed - both Carl Wilson

Turn under winter cover crops

If a winter cover crop of rye or Austrian winter pea was planted in the fall, turn it under a month before planting or seeding. I turned my winter rye under this past weekend adding “green manure” to the soil.

The large soil critters such as the earthworm pictured here (below right) and soil microorganisms will “chew” through the raw roots and tops over the next weeks for soil improvement. I left the soil “rough” and certainly didn’t step on and compact it. That’s an advantage of raised bed gardening – standing outside the bed to work your soil. It will not be tilled again when seeding or planting so the soil structure can be preserved as much as possible. More tilling would simply destroy it and there is no good reason for it.
















First, why do you need to wait a month before planting? The living critters working over the plants consume soil oxygen and can create plant health problems if not tilled in ahead of time. Once the bulk of the “raw” plants have been consumed, the soil environment stabilizes and lack of oxygen for plant root growth is no longer a problem.

Second, it’s about more than the buried plants. If it was only the plants we could find a way to chop them into fine pieces and instantly improve the soil. The secret is what the soil critters add to the mix and the squeamish need read no further. Simply know that without them, you couldn’t realize the soil structure and fertility improvement benefits from planting and turning under cover crops.

Large and small critters feed on the buried plants adding substances such as slime, mucus and fungal mycelia. Analyzing these substances show things such as gums, waxes and resins which glue soil particles together. Clumped particles enhance the tilth, porosity, and water holding capabilities of soil – all good things for healthy plant growth.

Take care of your vegetable garden soil and it will take care of you.


Photo credit: Two soil spading photos - Carl Wilson

Winter garden soil improvement

Vegetable garden soils ideally should contain 5% organic matter. If your soil is lacking, boost it during the winter season by planting a cover crop.

Cover crops are grown to produce green organic matter to turn under the soil. Along the Front Range, plant them from September to no later than mid-October. In addition to boosting organic matter, they prevent wind and water erosion over the winter, build soil structure and suppress weeds.

Legume cover crops can add nitrogen to the soil through the action of nitrogen fixing bacteria. It is good practice to buy rhizobium (beneficial root-associated or “rhizo” bacteria) and apply as specified if the seed is not rhizo coated. Rhizobium bacteria are specific for the legume species and are sold with a specific expiration date.

Broadcast the seed/rhizobium mix at the specified seeding rate and water to germinate. If irrigation systems are shut down, hand watering in the fall will help the crop establish before growth slows for the winter.

Winter ryegrass is often used alone as a cover crop (see photo). Winter rye/Austrian pea or a winter rye/hairy vetch mixture overwinter well in Colorado. Winter rye is a quick germinating and pioneering grass. Hairy vetch is a hardier legume than winter pea for the coldest areas. Seed winter rye and Austrian pea at 4 to 6 ounces per 100 square feet. Plan on 2 to 3 ounces of hairy vetch seed for each 100 square feet of garden area.

Note that these green winter plants are attractive to deer and geese. If they are well established prior to extreme winter temperatures, plants generally recover from winter grazing in spring.

When turned under, the decomposing green material can deprive spring vegetables of oxygen if not done far enough in advance of planting. Plan to mechanically till or hand spade under at least two weeks and better yet a month prior to planting. The legumes may have to be tilled several times to kill them and prevent resprouting.

Many gardeners do not have the land area to plant a cover crop for the growing season without depriving themselves of vegetables for a year. Winter cover crops solve that problem.

Consider planting a cover crop this winter to improve your soil.

New vegetable garden soils

So you've dug up soil for a new vegetable garden and are less than pleased. Heavy clays, low organic matter content and mixed top and subsoil layers are typical of urban Front Range soils around homes. What are your options?

Just like a human baby can’t do everything that an adult can do, a new garden soil can’t be expected to grow what a long-time garden with carefully cared for soil can. And don’t think that tossing a handful of fertilizer on these tough-to-garden soils will solve your problems. Poor physical soil conditions are the issue, not fertility.

Foot-long ‘Imperator’ carrots are likely not in your immediate future. Perhaps golf ball like ‘Thumbelina’ carrots are a more achievable objective. Better yet, rather than thinking about carrots that require loose soils for root development, consider fibrous rooted, pioneering crops such as lettuce, spinach and kale.

Another thought is to work on soil building your first year. Apply 2 inches of organic compost in the spring and mix in to the top 4 to 6 inches of soil. Plant a summer cover crop such as buckwheat (photo right), till it under in the fall and immediately plant a winter cover crop such as winter (annual) ryegrass. Let the roots of these crops do the work of penetrating these tough soils. Through their burial, add fresh organic matter to feed beneficial soil microbes and build your soil. Ideas on cover crops for Colorado conditions can be found in the CSU Garden Note #244, Cover Crops and Green Manure Crops.

Raised beds with hard sides (wood) or soft sides (soil beds photo left) are another option. Moving or importing desirable soil to construct these beds increases the depth of useable soil and jump-starts your efforts at building a good garden soil. Note that they too will likely require soil building measures such as compost additions and planting cover crops.

A final thought is to avoid overtilling your soil. Beating it up with a tiller is not the answer to building a good soil. Have a good reason to till (turning under a cover crop or mixing in compost for example) and don’t till frequently. Also till at moderate moisture content – not too dry and not too wet. Care for your garden soil and it will care for you.

How do you manage your garden soil? Comments welcome.