This past weekend at our Rutgers Ag Field Day in New Brunswick, NJ, I received many questions regarding the best time to plant vegetables. As the weather turns warmer and the days grow longer, gardeners are anxious to get their transplants into the garden soil. In our zeal to get started, it’s easy to forget that we are still in early May and that the danger of frost is still quite high.
Frost Free Dates for Warm Season Vegetables
For central NJ, it is wise to wait until mid-May to plant warm season vegetables in your garden. Plant timing for warm season vegetables should be delayed until the third week in May for northern NJ, and can be started the first week in May for southern NJ. These frost –free dates can provide guidelines but we must be aware that cooler temperatures and frosts can sometimes occur after these dates and damage sensitive plants.
Warm Season Vegetables that Need Protection
Many warm season vegetables originated in warmer climates such as South America. Plant origin often dictates the range of temperatures tolerated by plants within the same family. For instance, warm season solenaceous plants such as tomatoes, peppers, eggplant and potatoes or cucurbits such as cucumbers, melons, squash, and gourds grow best in the warmer temperatures of late spring to summer.
There is some variation in plant sensitivity to temperature within families. For instance, tomatoes can tolerate slightly cooler temperatures than peppers or eggplant. The group referred to as tender vegetables can be injured by light frosts and will grow very poorly in cool temperatures. The tender vegetables include snap beans, tomato, sweet corn and sweet potato. There is also a group of very tender vegetables that are the most sensitive to cooler temperatures and require the warmer soil temperatures of late May to early June for proper growth. The very tender vegetable group includes eggplant, pepper, cucumber, watermelon, muskmelon, lima bean, squash and pumpkin. The seed of very tender vegetables will rot and die in cold, wet soils.
Protecting Warm Season Vegetables
If a frost is predicted or temperatures in the mid 30 to 40 degree range, it is wise to cover plants for protection. There are many ways to protect plants from cold damage.
Frost Caps - If the plants are relatively small and compact, frost caps, buckets or other containers can be placed directly over the plants to provide some insulation. The container should be removed the next day before temperatures get too warm inside the container and cook sensitive plants.
Clear Plastic Wraps - Clear plastic can be wrapped around plants in cages to protect them from frost damage. The clear plastic works well around early planted caged tomatoes. The plastic allows light to penetrate during daylight hours and maintain higher temperatures during cooler days and nights. It is important to remove plastic on hot days to prevent damage to plants. Temperatures can climb inside of these plastic tubes under intense sunlight.
Water walls- There are various clear plastic devices that have long tubes connected in a circle. Once the tubes are filled with water, they will stand up on their own and provide a great deal of frost protection for early tomatoes. The water filled tubes heat up during the day and retain heat during cold nights to protect plants. These thick water walls provide effective insulation to counter cool temperatures. The water walls need to be staked to prevent them from blowing over with strong winds. After frost dates have passed, these devices can be removed.
Row covers are also sold in garden centers and can provide a marginal level of protection. Avoid placing row covers directly on plants, use hoops or sticks to hold the row cover slightly above the plant.
Moveable pots – Some gardeners plant tomatoes and peppers in medium to large pots and bring the plants inside during cold nights. I know of one gardener who places large pots on a home-made wood platform with rollers on the bottom and wheels his plants out during the day and back in his garage during cold nights. He moves his plants to maximize their exposure to sunlight and therefore maximize yields.
Warming Cold Soils
For those avid gardeners that demand early tomatoes, it is important to remember that even if we protect the top of the plant from freezing, the soils need to be warmed up to stimulate adequate plant growth. One way to warm cold spring soils is to use a dark colored mulch such as black plastic. Lay down black plastic mulch in the early spring to allow the daytime sun to warm soils. These warmer soils will jump start root growth on warm season vegetables crops to insure earlier harvests. For early tomatoes plant short season varieties that form smaller but mature tomatoes in less time.