Tomatoes are the most popular vegetable in South Texas gardens, but tomatoes are also one of the most difficult vegetables to grow. The intense heat of our summers, and the relentless Gulf winds, are great challenges to the gardener.
Tomatoes can withstand a lot of heat during the day, 100 degree temperatures or higher. But it is the nighttime soil temperature that matters. Tomatoes can only set fruit when the nighttime soil temperature is between 50 and 70 degrees. When the nighttime soil temperature rises above 70 degrees, the plant may live, but it will not make tomatoes. That’s why tomato plants often stop producing by July or August.
But you can cool your soil and produce tomatoes through the heat of summer with good mulching.
Mulch serves as a blanket of insulation over the garden: mulch prevents weeds, stabilizes soil temperature, and helps retain moisture in the soil. Good mulching can dramatically reduce the amount of water required for the garden.
Mulch also prevents the soil from drying out in our relentless Gulf winds: 90% of the microbial activity of the soil is found in the top four inches, the layer that dries out the fastest. Mulching protects this delicate layer of topsoil.
The best mulch material is always the most native, and the very best mulch of all is raked-up leaves from the trees on your own property. Native leaves have a special relationship with your soil. The leaves and soil have lived together in biological harmony for hundreds or even thousands of years. Their chemistry is highly adapted and they are perfectly matched for growing vegetables.
Native leaf mulch is available in vast quantities, usually right in your own yard, and even better, native leaf mulch doesn’t cost anything. Every year, millions of bags of freshly raked-up leaves are sent to landfills and transfer stations; leaves that would have made perfect mulch in backyard gardens.
On our farm, we have been using live oak leaf mulch for years. There is a myth that live oak leaves contain an acid that hurts plants, but that is not the case. Our gardens absolutely love their thick live oak leaf mulching. And as the leaves slowly decompose, they add structure to our garden soil and actually improve the soil Ph. We even use oak leaves in our compost piles, and as bedding for our laying hens.
The leaves of any native tree will work as mulch: oak tree leaves, pecan tree leaves, and even the duff, or detritus, of mesquite trees make wonderful mulch for gardens.
The thickness of mulch applied to the garden depends on the heat; the hotter the weather, the thicker the mulch should be. By the month of August, the mulch around our tomatoes is 6 to 8 inches deep. This thick layer of mulch keeps the soil cool at night, and our vines are full of ripe red tomatoes into the hottest part of summer.