Transplanting perennials is pretty straightforward if you get a few key things right – in theory, anyway. Sometimes, no matter what you do, a plant just won’t like being moved. Hopefully, though, the suggestions below will improve your chances of success.
Do your research. Some perennials simply don’t like to be moved. It’s not that they can’t, but it needs to be done carefully, and sometimes the risk is just too great. Plants with deep taproots, for example, can be difficult to dig up. Others are very forgiving. I had an abandoned clump of daylilies sitting on concrete for years, bare roots sticking out, soil gradually washing away, and they kept blooming! Some are very particular as to depth of planting, or they don’t flower well. For example, irises (a plant that responds well to being divided and transplanted) like the tops of their rhizomes to be even with or just below the surface. Before transplanting anything, it’s always best to know its growing requirements. Many plants require good drainage, a big concern in my clay soil, so I amend my beds with organic matter and plant those that are very sensitive to drainage problems on small berms or in raised beds.
Avoid transplanting when it’s hot (or even warm and sunny if the air is very dry and breezy). The ideal conditions are fairly cool, cloudy, and with rain expected in the next few days. (That said, many people recommend moving irises in July and August, after they’ve bloomed…there are always exceptions!) Also avoid transplanting when a plant is flowering, as that is an added stress. I do it sometimes, but I cut off at least half the flower stalks and any flowers that are setting seed. If I must transplant during hot weather, I cut off any flowers and also about a quarter of the foliage in order to lower water loss. Some people say you should cut off all foliage when transplanting, but this can trigger a plant to put resources into shoot growth at a time when they should be growing new roots. Transplanting too late in the fall may be a problem if the roots can’t get reestablished before the ground freezes.
Prepare your new hole before transplanting. Make it large enough that roots can be accommodated more or less as they are to begin with (you may have to enlarge the hole after you’ve got the plant out of the ground) . Don’t make it so deep that water will pass right through the root zone and collect below it. Even if you tamp the soil around the plant once it’s in, chances are it will be more permeable than the surrounding undisturbed soil. You can picture it like a colander with very small holes: when you water, you want it to wet the whole root zone (filling the colander) then “drain” into the surrounding soil. If the sieve is too deep, water could flow down without wetting the whole root system. On the other hand, if the drainage is really poor you might want the make the hole deep and wide so that excess water has a place to go, but generally wider is better than deeper. Another problem with a deep hole is that the plant will sink over time, and end up buried too deeply.
Amend your existing garden soil if necessary by adding compost, bagged soil, sand etc., but make sure it’s mixed with what’s already there or the roots will tend to remain confined to the hole – you are augmenting what’s there, not replacing it. They say gypsum helps with clay soils, though I haven’t tried it (soil amendment is complex enough for a post of its own). A small quantity of slow-release fertilizer can be added, but in general it’s not good to fertilize right after transplanting
Get most of the roots with the plant. Rather than using a spade for digging up a large perennial, I often prefer a garden fork (like a pitchfork, but with strong, flat tines). This makes it possible to loosen the soil in a wide circle around the plant without cutting the roots or having to uplift a really massive, heavy soil-and-root. Once the soil is loosened, you can use the fork to pry or lift the plant up, and with luck much of the root material will come with it, leaving the loosened soil behind. Or simply use a spade for the lift, digging outside the root zone. (Using a fork for any of this process may just be a personal preference, but I’ve found it handy is some cases anyway.)
How you replant is key! Perhaps the most common error is planting too deeply or not deep enough. Too deep and you risk stem rot, too high and the roots will dry out. It’s easy to make a mistake, especially since the plant can move in the process of filling the hole. First add a small pile of soil to the center of the hole. This is to help ensure there are no large air pockets under the roots. Lightly tamp all soil at the bottom of the hole. Put the plant in. I try to hold it at the correct height while I sprinkle loose soil (with no clumps) over the roots, attempting to fill the space without mushing the roots together too much. Fill about a quarter full and check height: however high the soil was on the stem when you dug it up should be even with ground level in the new spot. Maybe it’s being picky, but I sometimes get right down with my eyes at soil level to get it right. Adjust if it’s not – if too low, often you can raise the plant a bit and add more soil to the root area. Tamp lightly. Then keep filling, tamping at the halfway point a bit harder , and then again once the hole is filled. You want to make sure there are no air pockets and the soil is not too much looser than that surrounding the hole. Water, and check the soil height again; put in more if necessary. A shallow “saucer” around the plant to keep water where it’s supposed to be is OK, especially if the plant is on a slope. Finish with an inch or two of mulch, ideally keeping it out of direct contact with the stems. If the perennial spreads underground, mulch that is too thick can hinder this, and even a thin layer sometimes halts the spread of one that grows roots on aboveground stems. Some plants, like Dianthus, don’t like to be mulched at all because they are prone to rot
Sometimes plants moved from dense shade to sun will burn, and may need to be covered for a few days or a week with shade cloth, a light sheet, or something similar to block out the midday sun – even an inverted laundry basket would help.
Good luck and keep on growin’!
-Kristi