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WICHE: Shade devices slow bolting of spring vegetables

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By Jeneen Wiche

We usually don’t even consider breaking out the old window screens until July, but I just got in from strategically placing screens over my spinach, greens and Brussels’ sprouts.

It has undoubtedly been a hot spring: upper 80s in April with little deficit in rain fall calls for some early shade!

We are certainly used to protecting some of our plants from cold. Why not protect them a bit from heat?

Sure, we can’t change the ambient air temperature on a 90-degree day, but we can keep our vegetables shaded on the hottest days of the summer with reasonable results. How does it fell when you step into the shade?

The vegetable garden can start doing some funny things during a heat wave. When temperatures start to rise into the upper 80s in the spring, when vegetables crops are used to milder temperatures, we see what is called “bolting.”

Bolting typically occurs as the heat increases and the plant quickly sends up its flower because it suspects its life is near its end, which it is, if the heat continues.

It can make the vegetable bitter tasting, as well.

Watch any greens, spinach, peas and cole crops like broccoli, Brussels sprouts and cauliflower. They will make haste to flower and go to seed when temperatures rise.

The current weather may do them in for the season. 

Tender lettuces will bolt, too.

In the spring I plant more tender varieties such as Bibb and spring mixes, saving the more heat-tolerant for my second round in the summer garden.

Varieties more suited to the heat include the bolt-resistant Jericho, a Cos type called Pandero and a “heat wave” mix from Cook’s Garden.

The summer lettuces can stand some heat, but they will last longer if I alleviate some of the stress. This is where we can step in and help out with some strategically placed homemade shade devices. 

Providing a little shade will go rather far when temperatures rise into the upper 80s and 90s, and you may want to consider it throughout the garden.

You can use latticework, old screens, shade clothe, old sheets, fine netting or mesh (like what you would use for a wedding veil). 

Concrete blocks, bricks or terracotta flowerpots can provide the base for a screen to rest on several inches above a low growing crop such as lettuce. A picnic table bench works great as a prop for a screen set on a slant on the south side for crops that are medium-height, and using a shade cloth tied to four bamboo stakes can shade anything at any height from above or on a slant depending on the length and placement of the stakes.

You get the idea.

Unusually warm temperatures will also make plants transpire moisture through their foliage at a faster rate then they can take it up through their roots, which may cause wilting, but that doesn’t necessarily mean the soil is dry.

If a plant recovers over night, then you know that the wilting was caused by rapid transpiration, not thirst.

Many spring plants have not hardened off for summer quite yet, so this may occur in the landscape as well. Extra shade will help offset this, and if you haven’t mulched, now would be the time to reassess that decision in the ornamental and vegetable realms. 

Mulch helps keep the soil moist but it helps keep it a bit cooler, too. Most of us are experienced with a rainfall deficit, so be mindful to water all newly planted or seeded material.