Spring planting tips from a gardening meteorologist...
We all hear plenty about climate these days! Many of you know about microclimate too. One example is the urban heat island around D.C. which results in a longer frost free period; another is the infamous National Airport with its cooling river breezes (warming in winter). However, today I want to go even smaller scale. Let’s call it “nanoclimate”! It is something that nearly every gardener is going to run into at some time or another and the more you think about it the better off your plants are likely to be.
One of the first things you will notice when looking at plants in a nursery is the tags will often list the hardiness zone and many now also list the heat zone. A map of the hardiness zone can be found on USDA’s website. This map is a measure of how severely cold the winter can be and an indicator of what plants can be accommodated. In most cases you can grow anything that is for your zone or a colder (lower) one.
A heat zone map, on the other hand, is designed to show how much heat the area experiences and give a guide for which plants can withstand it. The American Horticultural Society came up with it and, while quite useful, is not yet universal. Washington D.C. sits in zone 7 in both classifications.
So does this mean you must always plant zone 7 or lower plants? Not necessarily, and this is where nanoclimate comes in. I have protected areas around the front of my house that benefit from its warmth. In addition, it is sheltered from the wind. I can tell you that in the winter, wind is a brutal enemy. Not only does it pull away any heat but it is usually very dry air which can desiccate evergreen plants as bad as a desert breeze in the summer. I have a magnolia that would like to have a word with me for planting it in a wind tunnel between my house and the neighbors. If I had talked to a knowledgeable nursery person or got in touch with a local master gardener, I could have protected that poor tree from some rough early winters. That is one area where I don’t have to dig up my dahlias either and they come back every summer despite books telling you otherwise.
Learning about your nanoclimate is sometimes trial and error. I often say that I spend more time moving plants around than planting new ones. Now that may say more about my lack of planning but sometimes a plant just doesn’t like where it is situated. I planted ferns on the north side of my house thinking that was the correct placement since it is shady most of the day. However, in the summer the sun angle still manages to provide them with three to four hours of sun and they are against the house siding which heats up like crazy in the early morning...moving time! In fact, evergreen shrubs like camellias and rhododendrons are best kept out of morning sun. In the winter, the plant will pull moisture out of the leaves and a quick warm up can put a lot of stress on the dehydrated leaves.
Then there are the moisture issues to contend with. I have a hill I grow plants on that like a drier environment, since there is a good deal of runoff. However, on the side of the hill not facing the sun, it is cool enough to stay damp during wet spells and hamper development of some of my dry loving plants. On the side of the hill where runoff waters collect and, of course, around the down spouts, a water loving plant is a great asset. Water irises and lobelias are great friends of mine in those areas.
Until two weeks from now, have fun in the garden and enjoy the beautiful displays of azaleas that only this area can so beautifully accommodate, especially along the recently saved hills in the National Arboretum...go take a look!
Capital Weather Gang meteorologist David Streit is also an active gardener. He earned a certificate in landscape design from the USDA Graduate School and volunteered many years at the National Arboretum.