Harden Off Plants And Tease Out Roots Before Planting Outdoors

So far it’s been a frigid spring. Plenty of rain, a few tantalizing days of sun, but for most of April we’ve been digging, dividing and transplanting like there was no tomorrow.

We’re moving ahead because the soil is finally warming and it’s time to get down to the serious planting. It’s time to break out the dibble, the trowel and the spade as well as the Planet Jr. Seeder and get things into the ground. The seedlings you started indoors are busting out of the pots, the cell paks at the garden center are calling you like the sirens of Circes, UPS is dropping off the bare root perennials, roses and fruit trees at your doorstep and that magnificent hydrangea that you read about and found at the nursery in a 10-gallon pot is on the back step pleading for a hole to be planted in. Oh, where to start and how to do it?

Well, let’s slow down for just a second and consider some of the physiology of planting. You can’t just take a nice leafy lettuce seedling from your indoor grow lights and plunge the plants into the soil outdoors. Even when your tomato plants are ready to move out or come home from the garden center, if you just pop them into the garden chances are they’re going to struggle. These plants need a short period of acclimatization otherwise known as hardening off. They need to be able to adjust to wind, direct sunlight and the vagaries of soil moisture.

A cold frame is the best method of hardening off plants, but I know that few of you have frames. You can buy plastic and polycarbonate structures that double as cold frames, but the trick is in knowing what to do with them. If they aren’t properly ventilated during the day, they become ovens. And on a chilly night, if they aren’t closed or mostly closed, the shock of one very cold night can set your plants back by weeks.

Remember that we can still get a frost out here late into April. The key word here is gradual. Plants going from the sheltered environment of a garden center, greenhouse or from indoor plant lights need a few days to get used to the dramatic change in conditions. If they are not afforded this period of change, they can suffer from windburn, sunburn and a sudden lack of soil moisture that results in instant wilting. At the very least they need increasing amounts of direct sunlight over a period of three to five days. One method of accomplishing this is to use row covers of finely spun thin sheets of white fibers that reduce solar radiation but still allows the soil and plants to warm during the day and retain some warmth into the night. The same thing is achieved by tomato growers who use hot caps, high tunnels or other insulating devices. A simple sheet of newspaper held down on the corners by stones can even suffice in a pinch. Even after hardening off. I prefer to plant on a cloudy day if possible to further reduce shock.

Then there are the roots. Cell paks, potted perennials and even some trees and shrubs grown in containers may have been in those confines for several months to several years. They are cramped, tight and cranky. Take a pot-bound azalea or cell-bound marigold out of the pot and put it straight into a planting hole and in no time you’ll have … nothing. It’s been shown over and over again that if these tightly root-bound plants (and remember it can happen to any plant be it an annual, perennial or 8-foot-tall tree in a pot) are planted without the roots being teased out, the roots will simply continue to entangle themselves and not venture very far from the original mass. The result can lead to the plant simply strangulating itself. But if you gently tease the roots and loosen them from the tight ball before planting, the plant is much more likely to thrive. Some root masses, most notably the fast growing annuals, can even be cut with a knife or sheers and the plants bounce back in days. However, this is not recommended with most perennials and woodies.

Most bare-rooted plants that arrive by mail need to be rehydrated and planted fairly quickly. Some may come with instructions to soak them in warm water for several hours. These plants—usually roses, young trees, brambles and many perennials—should be planted as per their instructions or they can be “heeled in” or potted until you’re ready to give them a final planting. But time is critical and the final planting should take place in days, not weeks, or there will be substantial disruption to the newly developing root system. Plant in a loose, highly organic soil and make sure air pockets are eliminated by fingering the soil before watering in. Never ever tamp the soil or step on it to firm it. Watering and time do the best job of settling the plant in.

Now there’s always the age-old question of how deep do you plant it? Here the answer is simple: You never plant deep. Cornell research that was published a number of years ago showed that for perennials, planting high resulted in more vigorous plants with substantially higher survival rates than plants installed too deep. The same will hold true of annuals and woodies as well. Especially in woodies, we’ve known for years that planting too deep is a major reason for failure of newly installed trees and shrubs. As a general rule of thumb, always try to plant at the same depth that the plant was at before. This holds true of plants taken out of cells, pots or field dug. Planted too deep, the soil becomes compacted, air is easily eliminated from the soil and all kinds of problems result.

Once planted and gently firmed in, most gardeners think it’s instinctual to fertilize. After all, newly planted plants need that extra boost to get going again and to get those roots growing. Well, a good shot of fertilizer at this point actually does just the opposite. New and very fine root hairs are just beginning to emerge shortly after planting and the salts in fertilizers can quickly and easily burn these root hairs and destroy developing fungi that are critical for root growth and nutrient uptake. Instead of feeding, you can encourage new root development with the addition of a biostimulant, including compost teas. Annuals can be fertilized lightly a few weeks after planting.

Supplying ample moisture at this point is critical. In all probability, you have reduced the root mass so there is less surface area for the remaining roots to take up water. All plants should be gently and thoroughly watered in at planting. Don’t drown them; just get the soil to the root depth moist, not flooded. Annuals will benefit from daily watering for up to a week, perennials a couple of times week for up to a month, while trees and shrubs may need watering for the balance of the season until the leaves drop or the soil gets cold in November.

Now get out and plant. Get some dirt under your nails and feel the good earth. Plant high, water well but don’t drown them and by all means, keep growing.