Perennial herbs play an integral role within the food forests that POP plants. Vigorous growing perennial herbs such as catmint, peppermint, and fennel serve as an understory to our orchards. The herbs act as a groundcover, outcompeting noxious weeds for the space, as a lure for pollinators and beneficial insects to create a balanced ecosystem, and increase the beauty of the space by bringing in vibrant colors, fragrances, and tastes into the orchard.
Some perennial plants go into dormancy as a means of survival through harsh conditions – both for the extreme cold of winter and the hot dry conditions of desert drought. The plants prepare their soft tissues for a water and nutrient shortage, slowing metabolic processes almost to a halt which to us looks like almost no growth, little water uptake, and often the loss of leaves in the fall. Plants that are native to such climates have evolved to not flower until they have gone through dormancy, so even indoor plants will need to go through dormancy if you want your plant to flower.
Perennial Herbs: Catmint, Chives, Lavender, Lemon Balm, Lemon Verbena, Oregano, Rosemary, Sage
Annual Herbs: Basil, Dill, Cilantro
In this article, we are specifically talking about perennial herbs. Perennials are plants that come back year after year. Annuals, on the other hand, die after one season of growth and need to be replanted each year (like tomatoes). If you attempt to overwinter an annual herb, it will likely be an act of futility. So while it is very tempting to try to save your large basil plant that tastes of summer, just harvest what’s left before the last frost and make a big batch of pesto. It will last longer that way, trust me.
Hardy vs. Tender Perennials
Tender Perennials: Rosemary, Lavender, Sage, Thyme, Oregano
Ideally kept in cool dry conditions, 40-60 degrees, with bright light. Low/no light is fine.
Tender perennials are plants that need extra care here in our borderline 6/7 growing zone. The most common amongst these hail from the Mediterranean – where the soil is sandy and dry, the air is heavy with humidity, and the temperatures remain temperate year round. Knowing where these plants come from gives us a clue of what they need to thrive. Rosemary is the pickiest amongst the group with lavender coming in a close second. Thyme, oregano, and sage are also native to the Mediterranean and are happiest in these conditions, but seem to be made of heartier stuff. The most ideal overwintering situation for these tender perennials is cool (40-60 degrees F), dry, and bright (though low to no light is ok). An unheated garage or shed is great if you have it (details below).
Hardy Perennials: Mint, Chives, Lemon Balm, Tarragon, Chamomile, Anise Hyssop, Bee Balm, Fennel, Catmint
Ideally keep outdoors for winter. Appreciate a long dormant season.
Hardy perennials I suggest you leave outdoors for the winter. They will be happier outdoors than indoors, and with a small amount of effort will be just fine.
Preparing for overwintering potted perennial herbs outdoors!
- Stop fertilizing your plants by the end of August. Fertilizing your plants encourages new shoot growth. Tender growth is very likely to become damaged during a cold frost, creating an entry way for pest and disease pressures to enter.
- Weed out pots by October.
- Prune hardy perennials after hard frost (down to 28 degrees F) down to 4-6” tall. Make sure any spent flowers and dead or diseased branches are removed. Save pruning to dry for winter use!
- Insulate! The roots of plants in pots are much more exposed to weather conditions than plants in the ground – they will overheat and freeze much quicker. So much so, that potted plants are considered to be two growing zones less hardy than the one that you live in. So insulating your potted plants is very important.
- You can dig your pots right into the ground so that the top of the pot and the soil line are even. Adding an extra 1-2” of soil on top of the pot is also good. Top with 4-6” of leaves, mulch, straw, etc. Be careful to select a location that does not sit saturated with water. Roots that sit wet too long will begin to rot.
- Alternatively, you can tightly gather (even stack!) your potted plants together in a location that is dry, blocked from the wind, and close to the house. The house should offer some residual heat and grouping the plants together will make them easier to check on and insulate. Once gathered, insulate/bury the plants with 4-6” leaves, mulch, straw, etc.
- Slow down watering schedule. Dormant plants take up much less water and do not like to sit wet so err on the side of too little rather than too much. Stick your finger an inch down into the soil and only if dry, then fully water. Never water when the temperature is below 40 degree F or if the soil is frozen.
Don’t be fooled – growing plants indoors is hard. Most herbs are not adapted to do well in the conditions our winter homes have to offer – low light and dry air. The amount of sun your plant receives sitting in the sunniest window in your home between December-February is equivalent to sitting in deep shade outdoors in June and July. Additionally, plants appreciate a humidity of at least 50%, our heated homes are much lower than that – just ask your skin. Plants suffering from low light and dry air become weakened and much more susceptible to disease and pest pressures.
For hardy perennials, I consider bringing them indoors for winter to be a method of harvest extension – allowing me to have a fresh herb harvest until the new year. After 1-2 months indoors, the plant is likely to become very leggy and depleted. But, it can be nice to pot up a small amount of chives or mint to have fresh for a few extra months.
Preparing for overwintering indoors!
- Repot in fall with fresh potting soil, especially if plant is becoming rootbound, or wait until spring.
- Prune out any spent flowers and any dead or diseased wood.
- Check for insects! If present, spray off with the hose. Wash leaves with soapy water. And if those fail, try using neem oil as directed on bottle.
- Move in gradually. The dry, low light conditions of our homes can be a shock to our plants as we bring them in for the winter. Plants in shock tend to wilt and loose leaves. You can ease the transition by doing it gradually. In the evening, bring plants indoors. In the morning, bring back out and place in the shade. Repeat for several days leaving indoors longer each day.
- Location. Place near window that receives at least 6 hours of sunlight a day, being careful not to allow any part of the plant to touch the window. Rotate plant occasionally as the plant is likely to become leggy as it stretches towards the sun.
- Air circulation. Stagnant air will allow mold and mildew spores to settle on plant. Use a fan to circulate the air several hours a day. If dust builds on plant, gently rinse off.
- Slow down watering schedule. Plants in dormancy use much less water and overwatering can cause roots to rot. Water only when dry when sticking finger into soil 1” – likely only once every 2-4 weeks depending on conditions. Create humid microclimates by misting plants 1-2 times per week, raising pot up on gravel and keeping covered in water, and by covering entire plant with a plastic bag to create mini greenhouse in times of crisis.
*** Overwintering in an unheated garage or shed that maintains temperatures between 45-60 degrees F throughout the winter is an ideal situation for tender herbs especially. Can be kept in darkness, but otherwise follow same guidelines as above.
This blog post was written by POP Education and Outreach Coordinator Corrie Spellman-Lopez.
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