It still comes as a surprise to many cold-climate gardeners that fall is one of the best times of the year to plant perennials. And yet, it really is!
There are a number of advantages to fall planting:
First, the ground is nice and warm and this root-encouraging warmth persists for many weeks even as air temperatures start to fall off. This is a huge advantage, as well-established roots are the foundation of healthy, mature perennials. It is interesting to consider that spring-planted perennials risk being set back due to cool soil temperatures, particularly if conditions are also wet.
Second, cooler air temperatures in fall make for less demanding watering and care needs compared to late spring and summer plantings - so, less work for you! Plus these plants are going into dormancy, so the only goal is to encourage root development, not top growth.
Third, perennials established in the fall are considered to be one full season more mature compared to spring transplants. This means fruiting plants are one full season closer to coming into production and flowering perennials will have more showiness and impact compared to the same plant established next spring. This effect comes from the well-established root systems of fall transplants, the opportunity for these perennials to emerge earlier than you could plant out next spring and the fact that most hardy plants require a period of cold dormancy to move them towards maturity. This third benefit of fall planting is actually a pretty big deal when it comes to measurable outcomes.
A fourth fall benefit that is talked about less, yet also has a large payoff, is the opportunity to establish new plants at a time when you can easily visualize your mature landscape, either because it is fresh in your memory or the landscape is literally still showy. By contrast, spring and early summer gardens are still emerging and it can be harder to figure out spacing etc as you add new plants.
Last, but super relevant, is the fact that the fall perennial budget goes further with plants on clearance. On the one hand, it may seem like clearance plants could be less reliable, but when it comes to well-tended perennials, looking a little scruffy is normal at this time of the year as the plants are responding to fall conditions. We display all of our perennials outdoors, so they are tuned into outdoor conditions (going from a protected greenhouse to the ground in fall is not a good idea).
Tips for fall planting...
Once you know what you want to plant and where, a little garden prep goes a long way. Start by weeding the space, reducing your chores for spring.
Once you have the quick weeding done, lay out your plants so you can visualize them in the new space, then start digging your planting holes.
The rule of thumb is to create a hole twice the diameter of the container and to the same depth. The current evidence-based best practice for establishing new perennials is to avoid adding lots of cushy amendments to the planting hole, instead encouraging the roots to establish in the native soil. Of course, soil building remains an ongoing goal - just not as part of the planting process. Bone meal remains a popular supplement for planting time, as slow-release phosphorous is the major nutrient associated with root development.
Once everything is ready, remove each transplant from its container and vigorously open up the root ball. This is the most important tip, regardless of the season you plant! If the roots are not opened up, not much will happen after you transplant. By crunching open the roots, you encourage root branching and make it easier for roots to wander into their new space. If roots are difficult to splay by hand you can use scissors or a sharp knife. Don't worry at all about damaging your plants... this process is extremely good for them (the only exception is where there is an obvious central tap root - don't cut through that).
Regardless of the season, watering is a critical part of the transplanting process. It pays to fill up your planting hole with water before dropping the plant in and backfilling. Then, water again once your plant is installed. As mentioned, the overall watering requirements for fall transplants will be less than those of spring/summer plantings. And, fall rains may take care of most needs post-transplanting. But do keep an eye on your fall transplants and water if they look wilted, if conditions get unseasonably hot or if rain is sparse.
We often get asked how late into the fall it makes sense to plant, and realistically, zone 3 hardy plants that are outdoors and in tune with fall conditions can be planted so long as the ground can be worked. However, by planting in earlier fall, perennials get a longer period in which to establish new roots before winter.
The other big question tends to be, "to cut back or not cut back?" This one is easy: don't do any pruning in the fall so as to avoid top regrowth ahead of winter. Leaving plants unpruned also encourages natural leaf and snow cover, which act as mulches protecting cold-climate plants from severe winter conditions.
Wishing local gardeners a few more weeks of puttering time in the outdoor garden!