Extending the growing season

Kurt Jones

County Extension Director

Our cats are always in a big hurry to get outside in the mornings, but have become a bit more hesitant in the past couple of weeks as the nighttime temperatures have been falling.  Probably more important than our cats’ comfort, is protecting the vegetable garden plants during these periods of cooler weather.

There are two types of frost that can affect our garden plants.  Advective frosts occur when a cold front comes through an area, causing temperatures to fall significantly below threshold levels.  Typically, our efforts to protect sensitive plants against advective frosts leave the gardener frustrated.  Last Saturday night (in Salida), we suffered from a radiation frost.  These frosts occur during calm, cool nights.  In mountain areas, our temperature inversions (cold air trapped in mountain valleys) can cause radiation frost damage to plants.  Frost protection is warranted in these scenarios, as temperatures tend to be just a few degrees below thresholds, and good soil warming occurs before and after frosts.

Some of the sensitive warm season plants (those needing protection first) include tomatoes, peppers, eggplant, cantaloupe, watermelon, winter squash and pumpkins.  Less sensitive warm-season plants include beans, corn, cucumbers, and summer squash.

When protecting warm-season vegetables from frost damage, the idea is to allow the soil to warm during the day, then insulate the plants during the night so radiated heat is trapped around plants and does not escape.  It is important to insure that the frost protection (plastic, blankets, etc.) do not touch the plants, rather that it floats above the plants.  The warm soil is the heat source at night.

Tomatoes are always of concern as our weather changes.  As the season draws to a close, many green tomatoes will still be on the vine. With a little effort, a temporary plastic greenhouse may be constructed over the plants to extend the season. Support the plastic so it doesn't touch the foliage. Ventilate to prevent excess buildup of heat during the day. Later, when frosts occur regularly, there will not be enough ground heat to prevent freezing within the shelter. At this time, harvest the remaining fruit, individually wrap it in newspaper, and store it in a cool place. As needed, fruit may be unwrapped and placed on a window sill to ripen.

Adventurous gardeners may enjoy having a cold frame to help with early season gardening and extending the fall growing season.  Facing the cold frame South with plastic, glass or plexiglass will provide good passive solar heating.  I lined our cold frame on the North side with gallon-sized plastic bottles filled with bleach water to help collect that solar energy and dissipate it throughout the cold frame as ambient air temperatures fall.  Putting in two tablespoons of chlorinated bleach discourages bacterial growth in the stagnant water in the jugs.  Paint the gallon jugs flat black to aid in solar heating.