Tough Weather Week for Vegetables

This past week was a tough one in Front Range vegetable gardens from several standpoints. For establishing tomatoes, we had five nights of overnight low temperatures in the forties F with a sixth night predicted tonight. The official early morning low on June 1 was 42 degrees F. [Postscript - the sixth night June 2 low was 39 degrees F - brrrr!]. This long stretch of lows under fifty degrees F will stop growth of tomatoes and take more days for growth to resume once lows are again in the fifties the first week of June. Can you say lost days for your tomato growing season?

Tomato transplants held for June planting 

Of course if you have your plants in nighttime heating water walls this has not been a problem. The other option some people have taken is keeping their plants "on the shuttle", bringing potted transplants in at night and setting them outdoors by day. June 2 will be the day to plant for those who have waited as this will be the first nighttime low over fifty F. If you timed growing transplants not to become too large, you will have success with holding as plants have stayed in active growth mode.

Remember tomato transplants can be planted deeper than they were growing in the pots and will root out along the stem. This only works if you have a deep depth of decent soil or use the horizontal-planting-with-upturned-top method in shallower topsoil.

Chances are very high the last spring freeze will have passed at this point. Remember the average last freeze in Denver is May 5 with the latest freeze occurring June 8, 2007. For kicks I'll throw in that the latest date of the last measurable snow in Denver was June 12, 1947 (data from NOAA National Weather Service Denver/Boulder office).

The second reason this was a tough week for vegetables was several days of winds in the twenty to thirty mph range. This dried foliage of newly established transplants or germinated seedlings. Careful watering and setting up wind protection structures were in order.

Photo credit: Tomato transplants held for June planting - Carl Wilson

Switch to warm season crops

It looks like the time to plant warm season vegetables without temperature modifiers (water walls, etc.) is finally here. Starting June 1, the forecast is for nights to consistently attain 50 degrees F and days to be in the 80's and even 90 degrees F. With the warm nights, tomatoes, peppers, eggplant and the rest of the vegetables mentioned last post will now prosper when planted.

This also means the end of cool season vegetables like the lettuce and bolting rapini (broccoli raab) pictured above. It's time for these to go into the compost as they are now most likely bitter or fibrous. Rapini in bloom does make a good bee plant if you want to encourage bees or are keeping bees. In tnat case you can wait until after flowering to pull plants.

Direct seed plants from a different plant family such as squash, beets and carrots or plant warm season transplants in place of the rapini.

Photo credit: Cool season vegetables gone by, Rapini blossoms - both Carl Wilson

When will the cold nights end?

Although this title seems like a song lyric, for the warmest of the warm season vegetables it's true. Tomatoes, peppers, eggplant, winter squash, pumpkins, watermelon and cantaloupe have more requirements to think about than not being able to take frost.

This group of vegetables grows well when daytime temperatures are over 60 degrees AND nights are over 50 degrees F.  It's this night temperature part that's problematic in high altitude climates like the Front Range.  The May 20 low was 38 degrees F and nights are predicted to remain in the forties for at least the next week. It likely will be June until they warm sufficiently for planting outdoors without additional temperature support.

That nighttime temperature support can come from water walls or perhaps a warm microclimate location with lots of stone or masonry to absorb and radiate heat. Otherwise the best approach may be to get on the shuttle - moving transplants out by day and indoors at night. This helps harden off plants for eventual transplanting anyway. Hardening off is toughening them up to lower outdoor humidity, wind and higher light intensities especially UV light not present indoors.

For what seems to be everybodys' favorite vegetable, tomatoes, it may be tempting to plant outdoors in a year like this when the apparent last frost was April 16. Stunted growth and susceptibility to pests such as flea beetle results. Plants can normally outgrow this pest when they are established quickly and get up and growing but are severely set back when stunted by cold nights and eaten by pests.

Waiting to transplant can often result in better growth and a harvest that arrives just as early as plants transplanted before the arrival of warm nights.

Photo Credit: Tomato that was transplanted when night temperatures were warm easily outgrows early flea beetle injury (see pin holes on lower leaves) Carl Wilson

When to plant vegetables

Warm La Nina effect weather along the Front Range this year has many people anxious to plant. In this post I want to discuss the common wisdom of planting by holidays and the science behind it. As always, individual decisions on planting date are based on risk tolerance. In some years early planting may prove a good bet and in others, an impatient action.

The holiday rule of thumb is to plant peas and lettuce on St. Patrick’s Day and peppers and tomatoes on Memorial Day along the Front Range.

The science behind this is that cool season vegetables are planted when soil temperatures are sufficiently warm for seed germination. These vegetables are able to withstand cold air temperatures. Warm season vegetables require warmer soil temperatures for seed germination and root growth, and warm, stable air temperatures for plant tops that are generally intolerant of freezing air temperatures. Many but not all warm season vegetables are planted as transplants and not direct seeded.

Soil temperatures for vegetable seed germination*

Cool season vegetables –

35 degrees - lettuce and onions

40 degrees – peas, radishes, spinach, cabbage

Warm season vegetables –

50 degrees – tomato, peppers, corn

55 degrees – beans

60 degrees – cucumbers, squash, eggplant

* soil temperatures measured with a soil thermometer at 4 inches at 8:00 a.m.

How does the planting by holidays prescription stack up with the science? It turns out to be a relatively safe guide when used in conjunction with observable weather trends.

Dates for soil warming for years 2005 to 2008*

40 degrees - Mar 28, Mar 28 and Mar 4, Mar 24

50 degrees – May 5, May 6, May 8, Apr 28

60 degrees – Jun 11, May 13, Jun 10, Jun 14

* recorded at the Lory Student Center weather station on the Colorado State University campus in Fort Collins (monitored by the Department of Atmospheric Sciences)

For more information on vegetables and their temperature tolerances, see CSU Extension Garden Notes Planting Guide #720.