So You Want an Indoor Herb Garden?

Let me start by saying what an indoor herb garden is not.

It is not a cute little collection of pots that support weak, spindly herbs that can give you at most a few leaves a week of production.

I don’t know about you, but I need a good handful a basil each week throughout the winter for pasta sauces, salads and many other dishes, otherwise it’s a waste of time and the windowsill space is better used by a houseplant.

To put it bluntly, if your indoor garden does not actually produce a reasonable amount of the herbs you need, then save yourself the grief of the whole exercise and just buy them at the grocery store.

Forgive me if I come off a tad direct here, but there are just too many blogs out there that merely sell the idea of an indoor herb garden – and how great it is to have so many herbs at your fingertips – without providing any practical advice. After all, what good is it to laundry list all of the herbs one may possibly grow indoors, without emphasizing the basic considerations that must be met to grow even a single herb species effectively?

I am not saying that creating an indoor herb garden is necessarily difficult, but it is not carefree-easy either; growing most houseplants is far more straightforward. However, the challenge lies not so much in the small things you do, but rather how you go about the basic set up and maintenance of your garden. I’m talking big picture stuff. I don’t care if you are hooked on chives or are a freak for rosemary. The overriding considerations in either case are the same.

Here are the 8 Golden Rules to follow when creating a productive indoor herb garden.

1) Light, Light, Light & More Light!

There is absolutely no question that insufficient light is the biggest reason why indoor herb gardens fail.

Think about it, the goal here is not merely to keep your herbs looking pretty in their pots, but to make them grow at a rate such that we can harvest a large portion of their standing biomass each week! Even if water, fertilizer and temperature are all optimized, this demands a very high rate of photosynthesis driven by high light, much higher than a typical houseplant will need.

How often do cut off 1/3 of your African violet to eat?

In addition, most herbs are very high light plants to begin with. Put them in even bright indirect light and they are bound to grow poorly and turn into a lanky mess, even if you aren’t routinely harvesting them.

So how much light do you need? Conservatively speaking, a solid 5 hours of direct natural sunlight is the bare minimum amount necessary to sufficiently power plant growth, with 6 or more hours being much better.

Just to be clear, I am talking about direct light – the kind that will warm your hand when held in it. Indirect light, no matter how bright, simply cannot be factored into this minimum requirement. Further, the pots should be situated right up against the window – i.e., within a foot or two of the glass (so long as the direct light reaches that far). The further you move away from a window, the more dramatically light energy is reduced.

Are there some herbs that can be grown in fewer hours of direct light? Yes, a few can probably limp along with 3-4 hours or so.  But your choices are so limited at that point, and growth will be so slow, that it makes little sense to bother at all. The cornerstones of most herb gardens, things like basil, rosemary, parsley, etc., all need the minimum of 5 hours of light just to grow well enough to allow a reasonable harvest from time to time.

The only place you are going to get this kind of natural sunlight is through a southerly-facing window free from shadows cast by obstructing overhangs, window dressings, surrounding vegetation, etc. I don’t care if the window is south-east facing or south-west facing, but a purely east or west facing window is just not going to cut it. Put a beautiful houseplant there instead.

And what if you don’t have a suitable south-facing window? The next best thing is using (or supplementing with) artificial lighting.

The two most common types of artificial lights for indoor gardening are florescent and metal halide lamps.

Florescent lighting is by far the most common and inexpensive type, and is the type used in various indoor herb garden growing kits, like the popular and hassle-free Miracle-Gro AeroGarden ULTRA. You can also forgo a kit and simply by the florescent lamp, such as the Hydrofarm FLT24 2-ft/ 4-Tube T5 Commercial System, which provides good light for a modest indoor herb garden.

Whatever you do, just remember that florescent lighting – even strong lighting – loses its potency very rapidly beyond 6 inches from the bulb. This short working distance is its biggest drawback. Consequently, to use it efficiently, it is advised to keep any florescent lamp within 4-6 inches of the tops of your herbs – and keep all of your herbs pruned to the same height so they can all benefit from the light equally.

As you might guess, another consequence of this short working distance is that the lower leaves of herbs grown purely under florescent lighting tend to grow relatively poorly; what you are really doing is growing and harvesting the tops.

If you use florescent lighting exclusively for your indoor garden, you will need to keep the lights on for at least 12 hours each day, with 16 hours being better.

For supplementing natural light, calculate the number of hours of direct sunlight received and then subtract this from 6; take the resulting number and multiply by 2, using the product as the minimum number of hours you should keep the lights on (do not use the lights while plants are in direct natural sunlight). For example, if your west-facing window gets 3 hours of direct light, you will want a minimum of 6 hours of supplemental florescent lighting each day.

Metal halide lighting, on the other hand, while a bit more expensive up front, is by far the better means of providing artificial light, especially if you plan to grow herbs in areas that receive little or no natural sunlight at all. These lamps are much more intense, and produce light that is usable several feet from the bulb. This allows them to penetrate  the plant canopy to encourage strong growth not only on the tops of the plants, but also down to the bottom leaves/branches of the plants as well. As such, you could use these lights to grow herbs that are over three feet tall, if they can grow that large.

In addition to providing a more natural-looking plant, metal halide lighting simply produces more vigorous and sturdy plants overall, with more leaves, shorter internodes (the distance between branches/leaves on the stem) and an all around bushier, healthier look. This also gives much more opportunity for frequent and substantial harvests. For example, with the Apollo Horticulture MH HPS Grow Light Digital Dimmable Ballast System, you can support a sizable indoor herb garden, and grow herbs indoors nearly as well as you could grow them outdoors in full sun!

The downside of metal halide lights, beyond the greater up front cost of the fixtures and bulbs, are their higher cost of operation (given their higher wattage) and much greater heat output. Unlike florescent lights, you generally need to keep a metal halide bulb at least 12 inches away from your plants to prevent burning them.

2) Choose Suitable Containers for the Indoor Herb Garden

Assuming you have met the light requirements, the next most critical decision is the growing container(s).

If you are relying on natural light, first examine the size/width of your sill. If it is very narrow (less than 6 inches), I’d suggest installing a shelf or using a table/rack or something else to hold the pots. You can certainly use a narrower sill, but you will be restricted in terms of the pot size you can use.

When growing herbs in soil (versus. hydroponically, which is a bit more complicated), the size and growth rate of your herbs will be limited in small pots, especially those under 6 inches in diameter. Small pots also dry out faster than larger pots, which could spell disaster if you decide to go away for a long weekend. To make the best use of windowsill space, try to find square pots.

In addition, I would strongly encourage you not to plant various herb species together in one container. Yes, I know planter boxes are cute and fit nicely on the sill, but they present a few serious drawbacks.

First, a common container requires you to treat all plants similarly, when species often vary in their preferences. The most notable point of variation is soil moisture. For example, due to its Mediterranean “roots,” rosemary does best in fast-draining soil that dries out a bit between waterings. Contrast this with parsley, basil and mint, which enjoy more regular watering and and a more organically-rich soil. If all of these species are grown together, someone will have to tolerate suboptimal conditions!

The second consideration is the spread of pests and disease. If plants are grown in a common container, you have little recourse when it comes to preventing the spread of things like aphids, spider mites, whiteflies, mealybugs and other parasites. If plants are grown separately, they can be removed from the window and treated individually at the earliest sing of infestation, thereby helping prevent the spread of pests while sparing nearby herbs the stress of treatment if not required.

Finally, separate growing containers allow you to simply change things up when necessary, without having to dig up and disturb the roots of surrounding plants.

Say you’re using a common planter and your parsley has fizzled out for some reason, but your basil and rosemary on either side are going strong.  You could dig up the plant and replace it; however, this is problematic.

Plants grown closely in a common container usually have co-mingled root systems, so if you dig up one of them, you will likely damage some of adjacent plants’ roots. This most likely will not kill them, as long as they are healthy, but it will certainly cause a temporary cessation of growth – similar to the shock associated with transplanting.

3) Start Herbs From Seed/Cuttings or Buy Young Plants

The easiest way to start your garden is with small, already potted plants, which are commonly available at most good nursery centers throughout summer and usually into the fall months. Don’t wait for the dead of winter to start your garden.

The next best method for quick-growing herbaceous species, like parsley, mint, basil, etc. is by seed. Of course, this takes a bit more time and planning, and if you have little experience sowing seed it may be more trouble than it’s worth.

If you are handy with propagating plants by cuttings, then feel free to do so. Basil, mint, rosemary and many other herbs strike root fairly easily; and most fresh herbaceous (i.e., soft, green) material sold at grocery stores can be rooted in water. For woodier plants, like rosemary, plant cuttings in a light seed-starting mix after treatment with a rooting hormone dip.

I do not recommend digging up outdoor plants as winter approaches, for a couple reasons.

First, these fall herbs are normally winding down their lives due to hormonal triggers set off by falling temperature and decreasing day length. For example, basil will be flowering at this time and often be reluctant to push new vegetative growth. This could also make using such material for cuttings more difficult. Exposing basil cuttings to long days (i.e., providing light for more than 12 hours each day) could help switch them over to the vegetative state.

Secondly, outdoor garden plants are likely to carry with them outdoor pests, which prove substantially more difficult to control indoors, where herbs are weakened by lower light levels. This is not to say that nursery material is always pest free, but they tend to be cleaner than most plants grown outdoors in the open soil.

4) Use a High-Quality Potting Soil in Your Indoor Herb Garden

Do not make the mistake of using soil in your backyard. Herbs grown in containers generally need a light, rich mix to support strong growth.

Depending on where you live, the soil in your garden is likely to be high in clay, which drains very poorly and can cause root root; or very sandy, which drains too quickly and possibly cause your herbs to dry out and die if you go away for the weekend. If you are experienced with plants, by all means make your own mix, but if you aren’t, just buy a bag of prepared potting soil.

Note that some brands of potting soils come with a fertilizer already mixed in. Always check the bag to make sure. If so, take note of how long the manufacturer says the fertilizer will last, since this will delay the onset of any feeding schedule, which we will be discussing later below.

5) Adopt Healthy Watering Habits

Overwatering or underwatering is a fine way to kill any indoor plant. Follow the general houseplant watering guidelines and “finger-stick” method I discuss here, which basically require you to develop a watering regime based on how fast your plants are drying out.

For herbs on a sunny windowsill, this will vary tremendously depending on the size of the plant, size of the pot, temperature, and the maturity of the plant. In addition, the ratio of plant to pot – i.e., more plant there is for a given volume of soil in the pot, the more water you will need to provide.

In addition, ensure proper drainage as I previously discussed for houseplants generally. This is especially important since, as we’ll discuss below, your herbs will require fertilizer; and fertilizer will cause the accumulation of salts in the soil absent proper flushing and disposal of the water drained from the pot.

6) Fertilize Your Indoor Herb Garden, But do so Sensibly

If your herbs are growing strongly and especially if you have been harvesting plant material frequently, they will need to be fertilized. Fail to do this and growth will begin to slow and/or your herbs will being to lose color and suffer from various deficiencies.

To produce new growth, a plant must tap a soil’s macro-nutrients and trace element stores to provide the raw materials needed for photosynthesis. Strong light without sufficient nutrients therefore does little good.

The loss of nutrients and trace elements is particularly important for herbs because they are being periodically harvested, thereby forcing them to produce more new growth that we will ultimately take again later. Without putting back these raw materials needed for growth, the small volume of soil most indoor herbs are grown in will rapidly become barren.

Because the vast majority of windowsill herbs are used for their leaves, rather than their fruits or roots, a general-purpose indoor/outdoor fertilizer with an emphasis on nitrogen is best. Personally, I like Osmocote. However, whatever you use, careful not to overdo it, as fertilizer burn is a always a risk in small pots. Follow the manufacturer’s recommendations and maintain a steady schedule as long as your plants are growing vigorously.

Very young plants/cuttings, plants that are weakened by pests or diseases, or those grown in lower light levels need less fertilizer to match their correspondingly lower growth rates.

7) Choose Your Herbs Wisely

I’d encourage you to give careful thought to start by picking at most three or four herbs that are your favorites – and that are either hard to get or expensive to buy at your local grocery store. You may even decide that there is just one type that fits this description.

The reason for this discretion is that it takes a fair bit of work to maintain an indoor herb garden, and space at the windowsill is usually at a premium. In addition, it is usually too much for the beginning indoor gardener to handle numerous herb species at one time. The fewer herbs you start with, the fewer you need to master at once.

For example, on my modest south-facing windowsill at this very moment are only two herbs – basil and lemon grass. I actually have two pots of basil, since I use this herb the most, and it can be quite expensive to buy it fresh in this area. And the single 6-inch pot of lemongrass (another hard-to-find herb) grows quickly enough to use for soups, sauces and cups of tea. That’s really all I need. Common herbs like parsley are dirt cheap to buy, and rosemary is a common landscaping shrub here in the southwest; no need to grow that indoors!

8) Avoid Extreme Temperatures

Most herbs don’t require any special temperatures when grown indoors.

Daytime temperatures of anywhere from 70 – 90F at the windowsill are fine, and getting sufficient heat is usually not a problem when growing in direct light, even in the dead of winter.

Nighttime temperatures of 65-70 are ideal, but most herbs can withstand nightly lows of 55F with no consequence.

The thing to be on the lookout for are cold drafts. If your windows have large gaps or otherwise promote drafty conditions, this could be a problem for your indoor herb garden, especially if you enjoy warm-weather herbs like basil, which are often the first plants to fall due to early frosts.

Featured (top) photo credit: “Kitchen herb garden” by Christine & David Schmitt under CC BY-NC-ND 2.0