Time for mid-summer planting

Vacant mid-summer "real estate"

Although warm season vegetables should be coming along well by now, don't forget about what you want to do with the vacant "real estate" in your garden from previous plantings.

April/May/June harvests of cool season and quick maturing vegetable crops often leave holes in the garden. While there is no problem with leaving ground fallow, do know that if planted before mid-July, a harvest can be gained yet this season. "60 dayers" (vegetables with 60 days to harvest or less) are what to think about planting now. Surprisingly, many cool season vegetables work well. Even though planted in the heat of summer, they will mature in cooler fall weather and be of good quality.

For more specifics on what to do to make a vegetable garden more productive from mid-summer through fall, consider attending my "Follow-on vegetable gardening" class at the Denver Botanic Gardens July 12 (click here for details).

Photo credit: Bare ground from harvest of spring crops - Carl Wilson

Time to plant hardiest seeds

The hardiest seeds (lettuce, onions and parsnips) can be planted out now that soil temperatures have reached 35 degrees F on the Front Range. Soil temperatures at the online Ft. Collins soil temperature website have been bouncing around between 36 and 39 degrees F since the beginning of the month. Soil temperatures warm on sunny, 50 to 60 degree days and cool in cloudy/snowy weather.

Parsnip seed is notoriously slow to germinate. It may work well to plant them with radishes to gain double use from the area. Quick maturing radishes can be harvested in 20 to 30 days and parsnips require the whole season (100 to 120 days depending on variety). 'Hollow Crown' is an heirloom variety (heirlooms are open pollinated varieties that have been in cultivation 50 years or longer). Other parsnip varieties are 'Lancer', 'Albion' and 'Javelin'.

Planting now means soil should have been fall prepped. Last season's dead plants should have been carted away or composted and the soil tilled and left rough to further break up during winter freeze-thaw. Any compost should have been added then. Note if you grew a winter cover crop, it will require 30 days after tilling under before you can plant vegetables.

Other spring crops require a 40 degree F soil temperature for seed to germinate. These include spinach, kale, beets, carrots, peas and more. Hold off on planting them for now. Of course if you have a soil thermometer and have reached forty at your Front Range Colorado location, proceed with planting.

Photo credit: Parsnip and Lettuce seed packets - Carl Wilson

It’s time to seed fall vegetables

Although it may seem that vegetable gardens have just begun growing well and planting is done, it’s time to seed crops for fall harvest. Sixty day crops planted in July will mature in September as weather is cooling.

Look at seed packets and note the days to harvest. Count back from the first fall frost date, October 11 for the Denver area. With cool season vegetables note that many tolerate the first light frosts of fall very well and an extended harvest period can be expected. Warm season vegetables must be planted to mature well before frost when temperatures remain warm.

Here are some vegetable planting suggestions listed with their typical days to harvest. Cool season vegetables such as peas (65), cabbage (85), collards (55), broccoli (65), kale (60), spinach (40), lettuce (60) and endive (45) are good candidates. Root vegetables such as beets (60), carrots (70) and turnips (50) can be planted this month. Even some fruiting vegetables such as bush beans (60) and cucumbers (55) can still be planted if done by mid-July. The very short season radish (30) can be planted into August with success.

Note that peas planted for fall harvest (photo above) are prone to powdery mildew. Choose powdery mildew resistant varieties. Be sure to provide support such as netting stapled on posts (right) for them to climb to insure good air circulation. Often harvests are reduced as compared to spring plantings. I’m willing to gamble on these Oregon Sugar Pod II’s because I like snowpeas and have a separate area where the air movement is not blocked by tomatoes or other vegetables.

Planting fall crops in “succession” where other spring crops have been harvested is a way to extend harvests past the first frosts and maximize yields from your garden.

Photo credit planted pea seeds and pea netting - 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.