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

Heat At Last

New grape growth

The average May 2015 temperature in Denver was 4.1 degrees below normal at 53 degrees F. The cool month slowed or delayed growth of some plants and postponed planting of warm season vegetables.

This first week of June turned hot with temperatures in the eighties F. With this warmth grapes finally began to grow in earnest. The late start means flowers will likely escape frost when they come into bloom shortly. This is good news for looking towards fall harvest.

Transplants of squash
and pumpkin vine crops

With increased sunshine soils have warmed over sixty degrees F and vine crop vegetable transplants can be planted.

Warm weather also means it was time to add water to the side channels of Walls O'Water (WOW) to push the tops open. This turns the structure into more of a cylinder than a cone and allows for ventilation while still keeping the plant warm at night.

Wall O'Water with more
water added to open top
into a cylinder

Keeping the WOW on tomatoes or other warm season vegetables is likely wise for another reason. May and June are active storm months for Denver and the Front Range meaning not only rainstorms but also the possibility of hail. WOW offer fairly decent protection from hail.

An understanding of temperature, it's effect on plant growth and what can be done to temper it is a must for gardening success particularly at our high elevation.

Photo credits: Grape growth, Vine crop transplants, Wall O'Water - All Carl Wilson

Still Useful to Plant in Wall O' Waters

Denver is receiving more sun and temperatures have warmed this week. That doesn't mean that it is ideal weather for planting tomatoes and other warm season vegetable transplants. Nighttime temperatures are still falling into the forties, far below the 50 F (and better yet 55 F) degrees nighttime temperature requirement for tomatoes. Note that the vine crops (squash, melons, pumpkins, etc. require 60 F degree nights).

A week after water channels filled at 
setup of Wall O' Water, sun has warmed water
sufficiently for planting within WOW cone. 
Plant deep for rooting along stem of this leggy 
tomato transplant.

So what's a gardener to do? You may have raised tomatoes from seed on your windowsill or purchased at a plant sale and the transplants are getting leggy. This is where Wall O' Waters come to the rescue. Even though often thought of for use in planting in the garden in April, they are still useful now.

Unlike mid-May when cloudy skies provided little solar radiation for heating the water in the tube walls of the Wall O' Waters, we are now receiving more sunlight. This provides warmth at night offsetting still cold night temperatures. Planting in Wall O' Waters also provides protection from wind and a sheltered environment for recovery from transplant shock.

Another benefit is protection from hail until plants grow above the walls. Even then plants pruned to the top of the Wall O' Waters by hail will retain enough undamaged plant within the Wall O' Waters to regrow.

If warm season transplants are planted in the open now without protection, they recover from transplant shock slowly and become stunted. We are assuming that night temperatures remain above freezing. They will require a week or more to recover from cold night stunting even when night temperatures warm above 50 to 55 degrees F in June. Although plants are leggy, you may be better off keeping them in pots and bringing them indoors at night until night temperatures warm if you aren't using Wall O' Waters.

See the manufacturer's website for more information on Wall O' Waters. Look to purchase them at your favorite local garden center.

Photo credit: Leggy tomato transplant for planting in Wall O' Water - 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

Warm Season Vegetable Transplanting and Plant Sale Recommendation

This past week we saw a week of rain ending with snow that dropped 3.3 inches of precipitation in my Denver garden. The wet week ended May 10th with an overnight low of 31 degrees F.

Two years in a row! Tomatoes in Wall
O'Waters on May 12, 2014.

Any early-transplanted warm season vegetables required protection with Walls O'Water or second best the use of frost blankets. Even so warm season vegetables will likely be set back and take time to resume growth. Cloudy weather during the week didn't allow much solar gain for Walls O'Water.

Planting thoughts this week should take into account wet soils. Give them time to dry to a medium moisture content before transplanting or seeding. Don't walk in or work soil when wet; you will only destroy soil structure if you do. Meanwhile, consider what warm season vegetable varieties you want to plant this year.

Vegetable transplants being grown
by Denver Master Gardeners for their
May 16 and 17, 2015 sale.

I'm impressed with the wide selection of vegetable varieties offered by CSU Extension Denver Master Gardeners at their spring plant sale this coming weekend. The sale is Saturday, May 16 from 8am to 3pm and Sunday the 17th from 10am to 3pm.

Location is the Denver CSU Extension office in Harvard Gulch Park, 888 E. Iliff Ave, Denver, CO. This plant sale promises to be worth a visit and proceeds benefit CSU Denver's education programs.

Click to download and then open this file for more information and a list of varieties offered at the sale: Denver CMG Spring Plant Sale.

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.

Fruit tree pruning time

Now that the worst cold winter blasts are hopefully over, dormant season fruit tree pruning should be completed in March prior to bud break. The pruned peach tree in the left foreground looks sparse compared to the unpruned tree to the right. This is as it should be.

Peaches put on a lot of growth and should be pruned hard. They bloom on 1 year old wood and if shoots are left for two years or more, they don't produce fruit. Much of last year's growth should be thinned out to avoid overproduction and allow light into the tree. Excessive fruit production reduces quality and also weighs down branches causing limb breakage.

To balance growth of fruiting wood with peach fruit production, remove one-half to two-thirds of last summer's growth. Space fruiting shoots 6 inches apart remembering to leave long shoots of 12 to 24 inches (they fruit better). On the interior of the tree smaller shoots can be left. Don't worry, you will have plenty of new shoot growth over the season to provide fruiting wood for next year's peach crop.

Little annual pruning is needed on fruiting sour cherries and plums. Bearing apples and pears require a light annual thinning for light penetration. Avoid removal of the short fruiting spurs. Unpruned trees may produce more fruit of lower quality for a few years, that is until growth gets so dense that fruiting on the interior ceases.

Train young trees for a sound structure in the first few years. Then maintain bearing trees with annual pruning for a productive fruit tree.

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