Keep cool about gardening during hot summer months

The days have been getting pretty hot recently. Not just the heat, but clear sunny skies and steady breezes have really dried things out. Diamond Head is brown, and at the University of Hawaii’s Waimanalo Research Station, the soil is bone-dry and rock hard with deep cracks.

It’s the beginning of what may be a very stressful summer for our plants (and those of us who work outside).

At the Urban Garden Center in Pearl City, master gardeners from the University of Hawaii offer a helpline service where home gardeners can call, email or visit with their gardening questions and samples of ailing plants.

Last summer was very dry, and many of the questions were related to heat and drought stress. Citrus trees died or lost whole branches due to a combination of drought stress and phytophthora (a fungal disease that can cause root rot as well as cracking bark and oozing sap).

Avocado trees exhibited leaf tip burn due to the heat, wind and a deficiency of potassium in the soil. One gardener had no pollen on her lilikoi, likely due to the high temperatures, which can affect pollen production and flower morphology in many plants.

Growing vegetables in the heat can be challenging, as many prefer cooler weather. For example, lettuce may tend to bolt (grow flowers, which makes the leaves bitter and smaller). Tomatoes can sometimes drop their flowers with intense heat, limiting fruit production.

When I grew kale and collards in Kaimuki, they wilted in the midday heat, no matter how much water I applied. The kale never grew well that summer and I had to pull the plants. Despite wilting, the collards were very tough and would bounce back once the sun got lower, so I was able to harvest pounds of nutritious leaves every week.

What can you do? Unfortunately, you will need to irrigate more frequently to keep vegetables happy.

Set up an automated system with a timer to apply water in the morning as well as once or twice during the hot part of the day. To reduce water use, keep your home and garden cooler with shade trees. Veggies like full sun, but during the summer they may appreciate partial shade, or having shade trees nearby to keep the surrounding area cooler.

When planting trees, make sure to give them enough space to grow and avoid infrastructure such as plumbing, electrical lines, walls and roofs.

HECO has a brochure called “Planting the Right Tree in the Right Place,” which has great recommendations. You can find the publication online at 808ne.ws/planttree. Pay special attention to the section titled “Plant Your Tree Properly” for the best tree-planting procedure.

Finally, maybe it’s a good time to find alternatives to fighting the heat. One option is to put your garden bed to rest for a few months with heat-tolerant, less-thirsty cover crops such as daikon, cowpeas, buckwheat and sunn hemp. You can find these online, sometimes at garden shops, at Fukuda Seed Store or Ko‘olau Seed and Supply.

Once these cover crops get started, they can thrive with a bit less irrigation than most vegetable crops, while recycling soil nutrients, suppressing weeds, and reducing plant pests and diseases.

Another option is to stay cool indoors. It must be a nice time for our producers who grow hydroponic greens indoors in climate-­controlled, air-conditioned facilities. You can do the same thing and grow your own sprouts or baby greens. Master gardener Christy McKinnon demonstrated an easy way to grow pea shoots indoors in reused plastic fruit clamshell containers or other containers.

Just fill the containers with an inch or two of potting mix, sow your presoaked pea seeds, and put them under a fluorescent light or near a window with filtered light. Keep the potting mix moist but not soaked, and in seven to 10 days you can harvest your pea shoots by trimming them at the base. You can even get another harvest from the resprouting shoots, though you will need to add an all-purpose fertilizer to give them more nutrients.

You can do this with sunflower, lettuce, baby greens, watercress, wheatgrass or any sprouts you see in the market. Make sure you use organic seed or seeds that have not been treated with fungicides. Use clean, potable water. Easy!

To learn more about gardening topics or to contact your local master gardeners, visit ctahr.hawaii.edu/UHMG.

Kalani Matsumura is a junior extension agent with the University of Hawaii’s College of Tropical Agriculture and Human Resources and coordinates the UH Master Gardener Program on Oahu.