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

Fruit tree freeze deaths

Subzero temperatures the second week in November did more damage than just preventing normal leaf drop (see Jan 16 "2014 Gardening Year in Review" post). As normal times for Front Range fruit trees to leaf out have come and gone, realization is dawning among home fruit growers that parts and indeed whole trees are dead. A warm fall did not allow trees to fully enter dormancy leaving them unprepared for the sudden, early subzero temperatures.

The row of sour cherries pictured have only a scattered bud or two alive. These few green buds will likely shrivel when summer heat arrives. Already gummy sap is oozing as a stress sign confusing some that peach tree borer is responsible despite the location higher in the tree than the soil line (no frass present either). See CSU Extension's fact sheet for information about Peach Tree Borer.

Few apple branches
 flowering/leafing on this tree

Not only cherry but in some cases generally hardier plum and apple trees show damage. In addition to fruit trees, damage of landscape shrubs and trees will become noticed more as the season progresses. Notable casualties to date include burning bush and spreading ('Manhattan') euonymus, rose, pyracantha, boxwood, privet, weigela, hibiscus, smoke bush, spiraea and buckthorn.

2014 Gardening Year in Review

Frost apparent on tomato leaves.

The best thing about 2014 was probably the extended growing season with the acknowledgment that many vegetable gardeners had to cope with hail along the way. Even though the first official freeze is recorded as October 3, lows weren't extremely cold on that date nor several times later in the month.

Temperatures dipped only into the low 30's F on Oct 3, 12 and 27 in Denver, well within the ability of frost covers to handle. The weather otherwise was warm enough to keep tomatoes growing late and certainly warm enough for mid-season planted greens and root vegetables to mature nicely.

This all came to an end on November 10 with a low of 13 degrees F followed by subzero F lows the next three nights. These temperatures are way beyond the ability of frost covers to handle and effectively ended the growing season for those using frost protection. True to life in Colorado's high altitude steppe climate, daily highs in the 50's and 60's were recorded the end of the month.

Cherry leaves retained through winter.

The unknown factor about this generally warm late growing season suddenly ended by subzero cold is the possible damage to fruit trees.  The most obvious sign of this now is the retention of leaves on apple, cherry and other fruit (and ornamental) trees. 

The cold obviously interfered with the normal leaf abscission process but less clear is how much damage might have occurred to buds and wood. This won't be fully known until spring when lack of bud break and dead wood will show us the extent of any freeze damage. Lack of gradually cooling weather to promote full development of dormancy can deal a crippling blow to fruit trees exposed to sudden subzero temperatures. Let's hope for the best.

Note for those interested in growing fruit trees, I will be teaching a new fruit tree growing class at Denver Botanic Gardens on March 14. See clickable link to DBG in the 2015 class offerings found in the right column.

Photo credit: Frost on tomato leaves, Leaf retention on cherry - both credit Carl Wilson

It's Cherry time

Mature, rarely pruned cherry tree SW Denver

Yields of cherries look reasonable considering the 28 degree F lows we had on May 11 and 12. (See May 12 post on fruit bud damage). Damage at these temperatures was expected to be about 10 percent. Birds also are always a threat and can strip trees of fruit in a few days unless trees are netted.

Tart cherries are hardier than sweet cherries which often winter kill in addition to flower freezing. Sweet cherries are comparable to peaches in this respect.

Recommended tart cherries are 'Montmorency', 'Meteor' and 'Northstar'. If you want to experiment with sweet cherries, try 'Black Tartarian', 'Kansas Sweet' or 'Stella'. Tart cherries are self-fruitful while sweets need a pollinator.

'Montmorency' is the standard tart pie cherry variety that produces a July crop of bright red, firm textured fruit. It is planted in the new (second season this year) "Le Potager" food garden at Denver Botanic Gardens (see photo).

DBG 'Montmorency' cherry

'Meteor' is a very cold-hardy tree growing 12 to 15 feet tall. It develops heavy foliage that can minimize problems with birds. Fruit ripens mid to late July.

'North Star' is a dwarf tree topping out at 10 feet. It sets generally heavy crops of fruit that turn dark red for July harvest.

Note that bush cherries are very hardy and also possibilities. Nanking cherry produces some of the first flowers of spring, can grow to 6 feet and fruit is harvested in July if birds and squirrels don't find it first. Sand cherry is another bush cherry reaching 4 to 5 feet and produces mild-flavored, deep crimson fruit. Both are self-fruitful.

Photo credit: Both 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

Year of added fruit tree maintenance

In many years untimely freezes leave fruit trees with little or no fruit on the tree. Not this year. Even though the first half of May was cool and had two more days than normal below freezing (7 total), the temperature on those coldest nights only dropped down to the 28 degree F limit during bloom and early fruit formation.

Limit because 28 degrees F is the lowest temperature that fruit trees generally can withstand before damage to the crop occurs. This year proved that right as there are bumper crops of most fruit including apricots, peaches, cherries, plums, pears and apples.

With a heavy fruit set come responsibilities. June is a good month to consider fruit thinning if the tree hasn’t taken care of this itself through the “June drop.” Peaches, apricots and apples should be thinned to 6 inches apart on the limbs when fruit is thumbnail sized. The photo right shows peaches in need of thinning (this should have been done when they were smaller). With apples, and pears this prevents codling moth worms from traveling between snuggled fruit and damaging two apples instead of one (photo below left).

Thinning helps fruit grow to a larger size and avoids later limb breakage from too heavy fruit loads. Cherries aren’t generally thinned. Be sure not to remove the spurs (short bearing stems) when you thin the fruit on pears, plums (not always thinned) and spur-type apples. Peaches bear directly on twigs with no spur involved.

For codling moth control to prevent wormy apples in apple and pear, thin fruit, trap insects and time insecticide applications appropriately. Insecticide applications should have begun right after petal fall. Permethrin and carbaryl (Sevin) are the most common homeowner treatments which are generally timed for 10 to 14 day intervals.

Click on this link for information on dealing with apple and pear insects.

Look here for information on managing peach tree borer, the most destructive insect of cherries, peaches and plums.

Photo credit: Heavy peach set, Apple cluster - both Carl Wilson

Banner year for fruit set

It looks like a banner year for fruit set based on an informal look at fruit trees and conversations with Front Range fruit tree owners. Apricots and peaches may set fruit in an average of 2 years out of 5 and they both have fruit this year. Later blooming cherries (see photo left) and apples bloomed when temperatures were above 28 degrees also escaping freezes when in bloom.

The danger now appears to be too heavy a fruit set. Peaches tend to set fruit too heavily and should be hand-thinned when fruit reaches 1 inch in size. Space fruit 6 inches apart (see photo). Apricot fruit should be thinned the same way and thinning will produce larger-sized fruit just like peaches.

In a heavy-set year, we inevitably get July and August phone calls from people whose home peach tree limbs have broken from the heavy fruit load. Remember that peaches only bear on 1 year old wood and bearing branches will be removed at the end of the season, leaving this year’s branch growth to bloom and produce fruit in 2011.

Apples are not always thinned but could benefit from thinning in the case of heavy set (photo left). Natural “June drop” is typical in heavy fruit set years but may not always thin fruit enough. Thin in June to 6 to 8 inches or to every other spur. Break up clusters leaving only single fruit. DO NOT remove the long-lived spurs, only the fruit. Thinning fruit produces larger fruit.

Sour cherries also have spurs that produce for 2 to 5 years. Clusters are not generally thinned. Sweet cherry trees are less favored for the Front Range and not as hardy as sour cherries and other stone fruits. Branches can winter kill.

Photo credit: Cherry bloom, Peach fruit thinning, Heavy apple set - All Carl Wilson

A Tale of Peaches and Potatoes

by guest writer Robert Cox, Colorado State University Extension Horticulturist, Arapahoe County

Gardeners may wonder why they cannot order certain peach trees from out-of-state nurseries. The catalog description may say that the peach variety “cannot be shipped to Colorado”.

There are two quarantines on peach tree shipments in the state. One prevents the shipment of white-fleshed or clingstone peach and nectarine trees into Mesa County, Colorado. This is meant to exclude peach mosaic virus disease from Palisade area orchards.

The other quarantine prevents the shipment of peach and related Prunus (cherry and plum) trees into the San Luis Valley. Peaches can be the winter hosts for green peach aphid that carries potato viruses. When peach aphids move to potato plants in the summer, the crop can become severely diseased and decimate yields.

Some out-of-state mail-order nurseries do not attempt to determine county of residence when a Front Range Colorado customer submits an order for a quarantined peach tree. The nursery may instead choose not to send a quarantined tree to any address in Colorado. While frustrating, little can be done but look for another supplier willing to recognize that your address is not in the quarantine area.

If it’s a small consolation, many states have similar quarantines on plants that could introduce diseases or insects to an economically important crop. Both peaches and potatoes are Colorado agricultural success stories and worth millions of dollars in income to the state.

[Peach photo credit, Carl Wilson]

Fruit trees on the Front Range

The biggest challenge with growing fruit trees on the Front Range is winter and spring “roller-coaster” temperatures. Fruit trees must tick off chill units to complete winter rest. Each variety has unique number of chill units. Warm temperatures above 55 degrees F will subtract from accumulated chill units delaying spring growth. Sound like this past winter? Once the rest requirement is satisfied, chill units are ignored until fall when they are reset by the next winter dormancy.

Following rest satisfaction, plants begin accumulating heat units for growth until full bloom. Again, each variety has a unique number of heat units (called growing degree hours or days). If this number of heat units is small and the tree is in full bloom when temperatures drop, blossoms freeze and the fruit crop is lost.

Finding fruit trees “programmed” with the right chill units and heat units for the Front Range is hard due to the unpredictability of our winter and spring temperatures (the “roller coaster”). While some trees may fruit one year, the crop is lost the next. With many types of fruit, realizing a crop one or two years out of five is typical. This is why there is no substantial commercial fruit industry here and instead it is found on the West Slope where temperatures are moderated.

What are the most reliable fruit tree varieties for the Front Range? Of course some that will grow here have other problems such as how susceptible apple and pear varieties are to fireblight disease. This is the time for readers to step up and tell us their experience. What type of tree fruit do you grow (apple, peach, cherry, plum etc.), what is the variety, what is your crop record (estimated number of crops in the last 5 years), and tell us your Front Range city?

Click comment below and let us know.

[Bowl of apples photo credit Carl Wilson]