Lifestyle magazines love to show pictures of fabulous herbs growing in color-coordinated containers on a winter windowsill. It just seems so trendy to grow your own herbs and serve them to your guests on a cold January day. But it’s not nearly as easy as the magazines would have you believe. Most people who write on the subject have obviously never gone beyond setting up a pretty display for the photographer, because the average windowsill herb garden goes downhill… fast!
Full Sun!
That’s because most herbs are full sun plants that really thrive only outdoors. Bring them indoors and they’re in trouble. It is nearly impossible to get anything close to full summer sun on a winter windowsill. The light is simply too weak. Plus indoor conditions during the heating season are brutal: the air is exceptionally hot and dry, while most herbs like nice cool nights. The result is weak, floppy growth, yellowed foliage and, more often then not, bugs. Yes, even if it is said that herbs ward off unwanted insects in the vegetable garden (and I’m not convinced of that), they definitely attract them indoors. Spider mites, whiteflies and aphids – the unholy Trinity as they have been called – seem to home in your poor herbs, leaving them even weaker than before. In fact, there really is only one herb that really does seem to adapt perfectly to winter conditions indoors: : the bay laurel (Laurus nobilis). Even then, watch out for scale insects and mealybugs.
Light Indoors Versus Outdoors
The PAR (Photosynthetically Active Radiation) outdoors in full sun typically ranges from 1,500 to 2,000 µmol/m²/s, while indoors, even in direct sunlight, it usually ranges between 50 to 300 µmol/m²/s, which is pretty much the same as shade outdoors. This significant drop in light intensity indoors is due to windows filtering UV light and reducing overall light levels. As a result, plants that thrive outdoors in full sun may struggle indoors without supplemental lighting to provide the intensity needed for photosynthesis.
Fluorescent bulbs provide PAR values of 50–200 µmol/m²/s, while LED grow lights offer higher PAR (200–1,500 µmol/m²/s). That being said, lights bright enough to grow most herbs indoors will seem quite intense and unpleasant indoors. Best to use them in a grow room!
PAR refers to the portion of the light spectrum (400–700 nanometers) that plants use for photosynthesis. It includes visible light from blue to red, which is crucial for plant growth. PAR is measured in micromoles per square meter per second (µmol/m²/s) and helps determine how much light energy is available to plants for photosynthesis. Unlike lumens, which measure light intensity for human eyes, PAR specifically measures light beneficial to plants’ energy needs for growth and development.
Under Certain Circumstances
Obviously, if you can offer wonderful conditions, you will obtain good results. Mint and parlsey, adapt well enough to bright indoors lighting conditions, but may need a humidifier to keep them from drying out. Rosemary (Salvia rosmarinus syn. Rosmarinus officinalis), grows well in a barely heated room in a very sunny spot… but this combination of conditions is not found in most homes. And almost any herb will thrive under intense artificial light and good humidity… hardly the average windowsill. If you do try herbs indoors under windowsill conditions, you can probably keep them alive for some time and maybe even get them through the winter, but they won’t be the thriving green plants you see in magazines.
The Easy Way to Grow Herbs Indoors
Do you really want to grow fresh herbs indoors over the winter? Here’s an easy solution: microgreens! Sow herb seeds in pots in a brightly lit, warm spot and harvest the plants when they are still very young, scarcely more than seedlings, well before they have time to suffer under the horrible indoor conditions. A monthly sowing from autumn through spring will keep fresh young herbs on your table throughout the cold season.
This text was first published on this blog on September 8, 2014. It has been revised and the layout updated.
Fascinating if discouraging! I cherish my outdoor herbs, and have never understood why I couldn’t grow them successfully indoors. You have erased so much guilt and frustration.
Another early morning of horticultural learning. Thank you. Ellen Asherman