A cold snap is forecast for this weekend, although temperature predictions vary slightly across weather sources. At the same time, some early plantings of watermelon and cantaloupe are already in the field in southern Indiana, or seedlings have already arrived and are waiting to be planted. What steps can growers take to protect these young plants and make a decision on whether to plant? And where to find reliable weather forecasts? These are the questions raised by a watermelon grower.
To address the second part of the question, I reached out to Beth Hall at the Indiana State Climate Office. Beth noted that she typically begins with the National Weather Service, which provides the foundational data used by most other forecast providers. Following Beth’s recommendation, I reviewed the National Weather Service forecasts and identified the expected minimum temperatures for Friday night. Based on the projections I saw on April 29, we would not anticipate significant issues for cool-season vegetables currently planted across the state. Warm-season crops grown in protected structures should also remain safe, particularly if growers close tunnels early to retain heat and use additional protection such as row covers.
Although the significant risk of frost/freeze across the state was low over the weekend, temperatures raised concerns for watermelon and cantaloupe growers. These crops are particularly sensitive to cold, and the large field scale often makes row covers or other protective strategies impractical. For seedlings that have not yet been planted, delaying transplanting may be the best option. In the meantime, store transplants in a well-ventilated area and keep them in relatively dry conditions to avoid excess moisture, which can create conditions favorable for disease development.
When to plant watermelons
The follow-up question is: when is the time to plant watermelons? In general, watermelons perform best when soil temperatures are above 70°F. For growers considering early planting, it is important to check both soil temperatures and the weather forecast beforehand. Soil temperatures should be measured in the early morning, when they are at their lowest. If soil temperatures are below 60°F or nighttime air temperatures are forecast to drop below 50°F over the next few days, the risk of poor establishment is relatively high.
Considerations for early planting
When it is not feasible to wait until soil temperatures reach optimal levels, the following practices may help reduce risks:
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Harden transplants before planting
Hardening is essential for melon crops. This process gradually exposes greenhouse-grown plants to outdoor conditions, helping them accumulate carbohydrates, strengthen cell walls, and promote root development. As a result, plants are better able to tolerate chilling stress. Begin hardening 1–2 weeks before transplanting by moving plants outdoors when temperatures are at least 45–50°F, and gradually increasing their exposure time each day.
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Use well-rooted transplants
Plant seedlings with a well-developed root ball. Plants of similar above-ground size can differ greatly below ground; those with a firm, fully developed root system are more likely to withstand stress and establish successfully.
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Consider deeper planting
Burying the hypocotyl while keeping the leaves and growing point above the soil increases the likelihood of survival. Even if leaves are injured by cold, plants can recover as long as the growing point remains intact. In contrast, if the hypocotyl is exposed and damaged by freezing, plant survival is unlikely.
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Grafted plants with squash rootstocks improve cold tolerance
Grafted watermelons onto squash rootstocks can tolerate low soil temperatures better than non-grafted plants or those grafted onto ‘Carolina Strongback’ rootstocks. In our experience with cucumbers, squash-rootstock grafted plants can tolerate soil temperatures in the 40s°F, while non-grafted plants often wilt at these temperatures. Additional information about the performance of the grafted plants under cold stress can be found in the experimental report Effects of Using Low Tunnel and Grafted Plants for Watermelon Production under Low Temperature Stress in Indiana.
What can be expected in fields that are already planted?
A common symptom observed after a cold snap is that seedlings in newly planted fields wilt and may eventually die. Under less severe conditions, older leaves may die while the growing point remains green. These plants can recover when conditions improve. The primary reason for wilting is reduced root function under low soil temperatures. Cold soils limit hydraulic conductivity in the roots, significantly restricting water uptake. As a result, wilting often becomes noticeable in the days following cold nights, especially under sunny, windy conditions, when low humidity increases water loss from leaves. Plants can recover if the cold period is brief and roots regain their ability to take up water. However, it is important to note that soils warm more slowly than the air.
Why do plants wilt in some fields but not others?
Even small differences in temperature can have a significant impact. Field location, topography, soil conditions, etc., influence temperature variation and, consequently, plant response. Another key factor is plant developmental stage. After transplanting, plants prioritize root establishment. Even if plants appear similar above ground, their root systems may differ substantially. Plants established just a few days earlier often have more developed roots and are better able to tolerate chilling stress compared to those more recently transplanted.
What can be done to improve plant survival?
Soils can absorb heat during the day and release it at night, providing a few degrees of protection to plants near the soil surface. To maximize this effect, growers may consider practices that enhance soil heat retention. In general, moist and firm soils absorb and retain more heat than dry, loose soils. Therefore, irrigating fields and avoiding cultivation before a cold event can help improve soil heat storage.
Several commercial products are marketed to help plants tolerate stress through different mechanisms. For example, anti-transpirants such as Vapor Gard® can reduce water loss, and plant growth regulator products such as X-Cyte™ influence plant stress signaling pathways. In addition, a wide range of microbial products and biostimulants are available, claiming to enhance plant resilience. The effectiveness of these products can vary depending on the crop, environment, and type of stress. Each product requires careful evaluation under specific conditions and within watermelon production systems. At this time, we don’t have enough information to support clear recommendations for their use.
This video on when to plant watermelons provides additional information about this topic.