When Can I Plant Tender Plants Like Tomatoes and Impatiens?

Is it safe to plant my tender flowers and vegetables like impatiens and tomatoes now?

When I first moved to Vancouver 12 years ago I checked the average last frost date of April 17 and decided to plant my impatiens and tomatoes about this time of year. Although there was no frost and they survived just fine, I found that their growth was very slow and in some cases almost stunted. I planted some more plants a month later which took right off and actually passed the earlier planted ones. I discovered that the weather seldom warms enough to stimulate normal growth on these plants that like warm growing conditions. So now I wait until mid-May to plant them or use covers to increase the temperature around them.

I should also warn those of you who live in the higher elevations surrounding the valley that the average last frost date can be as much as 3 weeks later than Vancouver’s official reading. If you live in the mountain areas I would definitely advise you to wait until late May to plant tender flowers and vegetables.

How to stimulate early growth.

There are two main ways to stimulate early growth by either warming the soil or the air space around your plants like tomatoes, peppers and melons. Plastic mulch is very effective in warming the soil around plants by 5 to 10 degrees. Red, green, brown and black plastic mulch comes in rolls or sheets 3 to 4 feet wide. Black plastic mulch absorbs the heat from the sun and passes it to the soil underneath. Colored plastic mulch allows the heat rays of the sun to pass through as well as absorbing the heat. Colored plastic is therefore slightly more effective than black. Experimental tests have shown the red plastic to be the most effective. All plastic mulches have the additional benefit of preventing weed growth underneath.

Plastic can be laid down over prepared planting soil with soil around the edges to hold it in place. Then X holes are cut so a trowel can be inserted to make a hole for plant roots. Larger plants are usually planted first and then slots are cut in the plastic to fit it around the plants.

A mini-greenhouse effect can be created by covering plants with clear plastic or floating plastic cloth row covers. Wire or electrical conduit can be bent into hoops for support. Floating row covers made from translucent plastic fabric are light in weight and can be laid loosely over plants allowing room for growth without support. Either can be held in place by using wire staples or soil around the edges.

Row covers trap the sun’s radiant heat around the plants and raise the temperature as much as 10 to 20 degrees. Floating row covers do not raise the temperature as much as clear plastic. However, because water can pass through them, plants are more easily watered without opening them.

Floating row covers are much more convenient for temporarily covering tender warm weather flowers such as impatiens, begonias, marigolds, zinnias and geraniums.


Allen Wilson

Allen has been writing about gardening for over 30 years. He is a retired professor of Horticulture.

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