Protecting Plants for Earlier Plantings

By Steve Reiners, Professor and Chair, Horticulture Section, Cornell University, Cornell AgriTech. This is part of a series of columns that he wrote about vegetable gardening during the pandemic that ran Spring-Summer 2020. Read more articles from 2020 | Read later articles in this series.

May 1, 2020

It’s been a chilly spring.  It’s still too early to plant warm-season crops like tomatoes, peppers, cucumbers, squash, and eggplant without some form of protection.  Frosts kill tender, heat-loving plants. Even temperatures in the 40s will bring growth to a standstill.  But there are several things you can do to get a jump on the season.

Warm-season crops do best when soil temperatures are in the upper 60s.  Right now, our soils probably are in the low 50s.  So, what can you do?  Cover your soil with sheets of plastic mulch.  That will warm the soil 5 to 10 degrees.

Black Plastic

For a garden, I recommend black plastic, sold in rolls 3 to 4 feet wide and varying lengths.  Simply prepare your bed as you normally do and stretch the plastic over the bed. This blocks light from the soil, so you won't have any weeds germinating underneath.  Simply make holes in the plastic and place your transplants.

Since rain will shed off the plastic, try putting a soaker hose or trickle tape on the soil before covering.  Then you can apply water directly to the roots as needed through the season.  Unfortunately, plastic mulch needs to be taken up at the end of the season and thrown away with the trash.

Notice, I did not recommend using an organic mulch like grass clippings, straw or paper.  That's because these will actually insulate the soil and keep it cool,  and that's the last thing we want to do for early vegetables.  We’ll add organic mulch later in the season.

Plastic mulch raises the soil temperature, but how do we raise the air temperature around plants?  You can create a greenhouse-like environment using plastic, row covers, or various items designed specifically to protect plants.  The idea of protecting early plants is certainly not new.  The French were using glass bell jars (called cloches) over their tender plants back in the 17th century.    By increasing the air temperature, the plants will not be bothered by cool daytime temperatures or nighttime frosts.

Household Items

What household items can you use to protect your plants?  Let's start with plastic, 1-gallon milk jugs to create hot caps.  Cut off the bottom and place the container over plants in the late afternoon.  Push it lightly into the ground so it doesn't blow away.

Keep the jug caps handy. It's a good idea to put them on each night to keep in the extra heat.  If you don't want to use old milk jugs, you can buy hot caps available in wax paper or plastic.  Simply place the hot cap over the plants.  A more expensive option would be Wall O’ Waters, a double cylinder of flexible plastic that becomes a rigid teepee when it's filled with 3 gallons of water.  Although much more expensive than the other protection devices, the manufacturer claims it’s the best since the water absorbs heat during the day and releases it slowly at night.

Which is best?  Researchers in Virginia tested milk jugs, hot caps, and Wall O’ Waters on tomatoes.  They found the highest day temperatures and greatest frost protection with the Wall O’ Water.  As for yield, the time to the first ripe fruit was reduced 11 days for the Wall O’ Water, 7 days for the hot cap, and 5 days for the milk jug, as compared to the uncovered plant.

Row Covers

All these methods involve covering individual plants.  Suppose we want to cover all our plants with a single cover?  For a garden, I’d recommend a lightweight floating row cover, which sort of looks like a white bedsheet (which you could also  use).  It's a lightweight fabric that can be draped over plants to protect them, not only from cool temperatures but also from insects.  Temperatures on a sunny day will be 10 to 15 degrees higher under the covers, and 3 to 4 degrees higher at night.

Keep row covers on until daytime temperatures consistently reach 70F, usually mid to late May.  You can reuse them again in the autumn as temperatures fall.  Keep the plastic mulch on the soil until the end of the season.  If you follow this advice, you can have fresh vegetables two weeks earlier than usual.

Next week, I’ll focus on home gardener’s favorite vegetable, tomatoes.