Like any good gardener, as you transplant your favorite seedlings from pot to the ground, one important factor you have your eye on is that "final frost" date for your region. After all, we're told over and over that frost can kill many sensitive plants, especially right after they've experienced the shock of transplanting. However, just because the final frost has come and gone according to the calendar, doesn't mean that your transplants are out of danger as even near-frost nights can result in wilting leaves, slow growth, and other signs of plant shock.
Even after that danger has technically passed, you may see a couple of nights when the temperatures dip below 40 degrees F. In these cases, there is a chance of frost even if it is unlikely, but the cold is also bad for certain varieties of plants. Here are some ways to protect your plants if you've accidentally transplanted them right before a "near frost" night.
Wrap or Cover
One of the most common ways to protect plants is through covering beds or wrapping trees. If your tree was recently transplanted, a near-frost night may cause leaves to wilt or make it harder to grow quickly; wrapping a blanket around the base of the tree may help to retain the ground's warmth overnight.
Covering beds is often done with garden cloth or black plastic; clear plastic, if exposed to the sun the next day, becomes like a mini-greenhouse that may create overly warm conditions for your plants. The black plastic and ground cloth, however, form a surface where any mild frost can form, rather than forming directly onto the leaves of your plants and harming their cell walls. One important thing to remember when you work to cover plants is that securing the covering to the ground with stakes so that air isn't moving extensively underneath the covering will do a lot to hold in temperature; if a breeze can pass through underneath, you may not reap as many of the rewards.
Remember that freezing temperatures that persist into the following day can create frozen condensation underneath a wrap or covering, so make sure that you remove the covers and wraps as soon as possible after sun-up.
In certain cases, the right move may be to spray an anti-transpirant, like Wilt-Pruf, onto the leaves of your plants. This spray essentially locks in moisture that might otherwise be lost during shocking circumstances like transplanting or a cold night or two. Pick a spray that is biodegradable, so it will naturally dissipate after protecting your plant for a little while. In addition to slowing moisture loss, some anti-transpirants also claim to decrease leaf and needle browning that can come about from cold and near-frost conditions.
These sprays may not do the trick for every plant, but for some, it will be a helpful way to avoid dangerous moisture loss. Just remember to read all the directions and, if you have a particularly sensitive plant to transplant, ask your local garden center for guidance before spraying or dipping the leaves of your plant.
Move Containers Near the House
Container gardens are particularly susceptible to frost, since cold gets into smaller containers faster than the broader ground. The good thing, however, is that most containers are at least somewhat mobile, compared to raised beds and ground plants. Come up with a plan to be able to delicately move your container plants if the weather is looking dicey. For larger containers, this may be as simple as getting the containers right next to your home, where they will get the benefit of the home slowly losing warmth to the outside environment, giving them a couple of degrees of extra warmth. For smaller containers, make a place in your garage, mudroom, or basement where the plants can weather the nights. It can be a pain to have to bring them in and take them back out in the morning, but it is worth it to keep them safe and warm.
Mulch Your Plants
For plants that are likely to do fine in the cold but which still worry you a bit, a nice in-between choice is to mulch. Not only does mulch help with keeping your plants well-irrigated later on in the year, when bare soil can allow too much water to evaporate in the summer sun, but it also keeps the temperature of the soil more constant. Near-frost nights have to chill down the mulch before they get to the soil and roots of your plants, so your plant's hardiness will be a little higher. Just know that mulching isn't likely to protect delicate leaves if an actual frost comes along. For this reason, mulch can be good when used in combination with an anti-transpirant spray or a great option when your trees or shrubs don't have big leaves yet.
No matter what you want to do to keep your plants safe as they transition to full-on Springtime, make sure that you make plans for the next day as well. Your solutions are only as good as your follow-through, and wraps and covers should be removed if they will create excessive heat the next day.