Spring freezes and fruit trees

Peach bloom April 1, 2017

A recent look showed peaches in full bloom in Denver during a week in which night temperatures are expected to drop to the mid to low twenties F.

Spring freezes during bloom are the biggest concern for home fruit growers.

While a dry March had few cold nights, April is shaping up to have much more variable weather. This includes rain/snow storms followed by cold nights due to radiational cooling after storms pass and skies clear.

USDA Hardiness Zone 6 or better yet Zone 7 (Colorado's West Slope) are better areas to grow peaches than Zone 5 Denver. While there are Zone 5 peaches, many backyard growers don't hunt for and plant them.

Yellow Delicious apple
bloom April 1, 2017

Peaches aren't the only trees with blossoms out now. This Yellow Delicious apple tree is well on its way to having flowers out during this week's expected cold nights. Yellow Delicious is a medium chill apple (600 to 700 chilling hours), fireblight susceptible and not on my list of recommended varieties for the Front Range.

Honeycrisp is a medium to high chill apple (800 to 1000 chill hours) that is more fireblight resistant and on my recommended variety list. As you can see in the photo, it breaks bud later than Yellow Delicious.

Honeycrisp apple branch
March 30, 2017

What can you do with a tree in bloom when frost is expected? Homeowners with young (short) or dwarf trees can throw a plastic cover over them and use a heat source underneath. While there may be some heat in the ground to trap after a warm March, you will have to supplement as soils aren't that warm yet.

Lights with old-style incandescent bulbs or any bulbs that produce heat will work. Do be mindful of fire safety when using lights under covered trees and remove tarps the following morning to avoid overheating trees on sunny days.

See the CSU Extension Garden Note 722 "Frost Protection and Extending the Growing Season" section on Lights for Addtional Heat for a description of using plastic covers and Christmas lights for warmth. Does anyone have another favorite way to provide supplemental warmth under a covered fruit tree that they want to share?

Photo credit: All photos Carl Wilson

Fruit tree bloom could be early?

Apricot 'Sungold' Mar 11, 2017

This week's forecast calls for weather not just in the sixties, but reaching into the seventies and possibly even eighty degrees F by the coming weekend. If the warm weather we've been having causes early bloom of fruit trees, blossoms could be caught by freezes.

Apricots are notorious for early bloom. Pictured is Prunus armeniaca 'Sungold' at Denver Botanic Gardens that was in full bloom this past weekend. Apricots may only bear fruit one year out of five (?) here on average because blossoms get caught by freezes in late March and April. The good thing about apricots for a homeowner is that they make a handsome ornamental landscape tree even if there is no fruit crop.

'Sungold' apricot bloom closeup

The bigger concern is with our other fruit tree species that may start to break bud as chilling requirements are met and temperatures allow them to resume growth. They need moisture to do this as well.

Along with cold temperatures, moisture has been lacking so far this spring. Watering your fruit trees is likely a good idea on the Front Range if the current dry conditions continue. Thankfully, our mountain snowpack is now around 130% of normal so we have water to irrigate.

Photo credit: Both apricot photos Carl Wilson

Good year for fruit on the Front Range

It is often said that spring freezes are the biggest danger to fruit growing. This year many areas of the Front Range were spared freezes at flowering resulting in generally good fruit set.

Another reason for heavy fruiting is that stored energy is high because many trees bore little fruit in 2015. A warm fall in 2014 didn't allow many trees to prepare for cold weather. Sudden subzero temperatures in early November 2014 following the warm fall caused freeze injury to a variety of plant tissues some affecting flower buds. That and usual spring freezes resulted in generally light crops in 2015. Not using energy on fruit last year left trees with ample stores to carry a large crop to harvest this year.

It will be a good year for gleaning to donate to food pantries but do check with landowners first.

This street-side apple in Louisville, Colorado also illustrates another point. Trees sited in heavily irrigated and fertilized lawns often respond with excessive vigor. They show lots of shoot growth and poor fruiting. This apple in an obviously sparsely irrigated and little fertilized area is bearing a nice crop as pictured in the close-up above.

The take away message is that mature fruit trees do better on less water and fertilizer than is applied to grow medium to high quality bluegrass lawns. A separate non-lawn site for growing fruit trees is a better growing situation for producing fruit rather than leaves.

Photo credit: Both apple tree photos credit Carl Wilson

Prospects for Fruit this Year in Denver

The 2015-2016 winter in Denver was unusually mild for fruit tree flower bud survival with only two days of zero degrees F (December 17 and 28) and no temperatures below zero (National Weather Service DIA records).

February 2016 brought enough warm days that fruit trees with a low chill requirement and low growing degree hours to reach bloom such as apricots and peaches were in flower by mid March.

Apricot in bud March 5

Apricot in flower Mar 19
with snow on branch

March brought fifteen days with minimum temperatures 28 degrees F or below. Overnight lows were 20 degrees F on March 19th and 10 degrees F on March 24. As a rough guide, 28 degrees F is the low temperature where many flower buds showing color or in bloom can be damaged. Prospects for apricot and peach fruit this year are likely poor.

Apple blossoms April 24

April has brought warmer nights. Since April 1st when the overnight low was 25 degrees F, the minimum temperature has not dropped to 28 degrees F.

This means that late blooming fruit such as apple and pear, and even fruit that bloomed slightly earlier such as sour cherry and plum may have fruit this year. Note that the specific location of fruit trees, localized overnight minimum temperatures, health of trees and amount of bee pollination activity are some of the factors that will affect your prospects to develop blossoms, set and grow fruit.

Photo credit: All photos credit Carl Wilson

Cool, Wet Weather Persists

Freeze damage on tender grape leaves

The limp grape leaves in the photo show one effect of cool, rainy spring weather. Clearing skies on May 10 as a storm system moved out after a week of rain allowed temperatures to plunge below freezing overnight damaging tender new leaf growth. Reserve buds will need to grow to take the place of the lost leaf tissue. Leaves that had already fully expanded and hardened were not damaged.

If you are anxious to plant warm season plants this is a lesson in hardening them off first. Plants purchased at plant sales should be shuttled in and out of the house or coldframe for at least a week, out on warm days and in overnight or on cold days. Nighttime temperatures remain in the forties at mid-month, way too cold for tomatoes and the like. Soil preparation including fertilization and adding compost are good places to direct your efforts now.

Spreading compost on vegetable
garden to mix in 4 to 6 inches

Soil temperatures at 4 inches have crept up to 54 degrees F after plunging to 50 degrees F following a week of cloudy, rainy weather. Keep in mind that the minimum soil temperature for tomatoes is 50 degrees F. Planting is possible under Walls of Water that keep above-ground plant parts warm overnight. Better yet use them in combination with plastic mulch that also utilizes the sun to warm soil for roots to grow. The plastic can be removed in June along with the Walls of Water if you care to not leave in place for the season.

 For some vegetable gardeners warm season crops are the reason to grow a vegetable garden. Cool, rainy spring weather has made this more of a cool season vegetable gardener's year so far.

Delayed planting of warm season vegetables to closer the end of the month is likely a better approach for those not using Walls of Water and plastic mulch. For information on planting warm and cool season vegetables, see the CSU Extension "Vegetable Planting Guide."

Photo credit: Freeze damaged grape leaves, Compost application - both credit Carl Wilson

Fruit bud damage after the storm

May 12, 2014 snow on cherry fruit buds

Cold damage to plants is often much greater the day following a storm. This is when skies clear and without a cloud blanket to hold heat in, radiational cooling comes into play. At high Front Range Colorado elevations with low humidity, plant cold damage from this type of cooling is common.

At this stage of tree fruit bud development (full bloom to petal fall), we can expect 10 percent bud kill at 28 degrees F and 90 percent bud kill at 25 degrees F. This is with a 30 minute exposure and it doesn't matter if it is apples, cherries or peaches. European plums are somewhat hardier and it will have to go down to 23 degrees F to reach an expected 90 percent fruit bud kill and similarly to 24 degrees F for pears.

Cold damage to developing buds of fruit trees can be minimized by planting near the tops of slopes where cold air drains to lower levels and by avoiding blockages (such as fences) to cold air draining away .

Good yields of tree fruit may only be achieved in 2 or 3 years out of five on Colorado's Front Range. This is looking like it could be one of those off years.

Photo credit: Cherry fruit buds in May 12, 2014 snow - 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

Hard freeze strikes late

When the first widespread freeze finally came, it froze hard. Officially it was 20 degrees F at 6:30 a.m. at DIA. Considering the average first fall frost is October 7, the 28th of the month is nothing to complain about.

Hopefully you rescued what you could from the garden before the freeze. Somebody commented to me that tomatoes they’ve harvested in the last few weeks have had little flavor. Forty degree nights are flavor killers for this warm weather vegetable. It’s a good lesson in harvesting earlier for fresh use, or using the late proceeds in soups or sauces.

The squash and pumpkins should be in by now. This Carnival acorn is kind of spooky looking for Halloween and pretty ornamental in the garden. Good flavor too.

All that remains tucked in to the side of the frozen tomatoes are the cold-hardy greens such as kale, and the root vegetables such as carrots and beets. They will be good for another month or more. The root vegetables are particularly adapted for in-ground storage late into January if they are mulched.

Photo credit: Tomato harvest, Carnival acorn, Frozen tomato and pepper plants – All Carl Wilson