Slow Start to Warm Season Crops

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Are your summer vegetables off to a slow start this year? The following will explain why this has happened, and what you can do to alleviate the problem.

A persistent cool, cloudy, and wet, wet, wet weather pattern has caused a lot of hair-pulling for growers, vineyard managers, and home gardeners. These trends make it especially challenging for warm-season vegetable crops to get established. Corn, summer squash, tomatoes, okra and melons need soils above 60 F to germinate and then warmer temperatures and sunshine for rapid growth.

I received several calls this spring about poorly growing plants. “My corn is small and turning yellow?” “Why are my squash NOT growing leaves, but putting out flowers and fruit?” Cold and wet soils early on can delay plant establishment, cause stunting, and root disease. An underlying driver is slow release of essential nutrients in cold soil. Soil microbes are beneficial soil critters needed to break down and release nutrients from soil organic matter. Microbial activity is low in cold soils, and they aren’t processing nutrients into plant-available forms.


Plant growth slows when ANY essential nutrient is limiting, either because roots can’t take it up or it is tied up with the soil. When a root system is unable to extract nutrients from the soil, the deficiency can appear in plant leaves. Each plant nutrient has its “signature” deficiency symptom. When nitrogen N is deficient, the lower leaves turn yellow while veins remain green. This often shows up in heavy feeders like corn and cucurbits (squash, melons, and cucumber). Corn also has a relatively high sulfur (S) requirement. S deficiency resembles that of nitrogen, except yellowing occurs in the younger leaves, top part of the plant. I have likely seen both in sweet corn this year!

Phosphorous deficiency

Phosphorous deficiency – photo by author

Chilly soils also limit plants’ ability to take up nutrients. Waterlogged soils inhibit root growth which is important for plants to access phosphorous and micronutrients. Phosphorus (P) promotes vigorous root growth and is critical for plants to establish a strong root system during early growth. P deficiency is common in cool, wet soils because of reduced P solubility and slow root growth. Reddening or purpling of leaves is a classic symptom of P deficiency. The effects of low P can seldom be fully corrected after emergence.

Be wary of diagnosing a nutrient deficiency because they can be easily confused. Damage to plant roots, insects, and/or disease can look similar. A leaf tissue sample is needed to get a definitive diagnosis. Soil tests are an accurate tool to identify nutrients that are low in the soil and may cause a deficiency in the plant.

Managing crops in wet and cold soils

This year serves as a good reminder why applying starter fertilizer before planting is important. A starter fertilizer is a small amount of fertilizer placed close to the seed or seedling at planting to give plants a robust start of growth. Complete fertilizers with both N and P make excellent starter fertilizers. For optimum effectiveness, use fertilizer with ammonium combined with phosphate such as Monoammonium phosphate (MAP; 11-52-0) and ammonium polyphosphate (10-34-0) based fertilizers. P deficiency will often show up in cold springs, even with starter fertilizer, but the plants won’t be as affected if they have a good shot of it to grow on.

Ways to improve crop establishment are planting into black plastic and cultural practices that remove excess moisture from the soil. Soil temperature is strongly influenced by soil moisture. Clay soils hold more water than sand and tend to run cold. Light tilling, planting a cover crop, and growing in raised beds all improve soil drainage, with the added benefit of warming the soil.

The good news is that with the warm weather the week, plants may grow out of nutrient deficiency and stunted growth. Give your plants a boost with sidedress fertilizer, especially a high N product such as 16-4-8, 34-0-0 or 21-0-0. Some N was released in the soil during warm spells this spring. This mineral N was likely leached by heavy and persistent rainfall in the past few weeks. In general, most vegetables benefit from a sidedress fertilizer 1 month after planting. Home gardens can apply 1 Tbs of a high N fertilizer per plant. Incorporate with a trowel 4” to 8” from the plant to avoid injuring the roots. Water-soluble fertilizer can be injected through a drip system. Organic fertilizers, e.g., composted manure, blood meal, feather meal, supply small amounts of nutrients over a long period of time. As a sidedress, they will have few immediate benefits. They add to soil organic matter which promotes long term storage and capture of nutrients in the soil.

If your first vegetable plantings are struggling, this year may be an excellent time to try succession planting. It’s not too late to make 1 to 2 more plantings of snap beans, cucumbers, and summer squash through the end of July. This will extend your harvest, and provide backup if earlier spring plantings aren’t successful. If you have questions about fertilizing, soil testing or plant issues contact the N.C. Cooperative Extension, Yadkin County Center at 336-849-7908.