First Published 2/19/2004
Humidity

Generally speaking, gardeners don’t pay too much attention to humidity; there’s enough to worry about, maintaining the right balance of sunlight, soil moisture, and fertilizer. But when we see what our desert-dry winter air does to our skin, you can’t help but wonder what effect humidity has on our plants.

It’s All Relative

There are a couple of ways of expressing the moisture content in the air. Absolute humidity measures the number of grams of water per cubic metre of air – a precise but unwieldy concept for most of us. The more common way to measure moisture content is relative humidity, or R.H. The word “relative” alludes to the fact that the amount of moisture the air can hold depends on its temperature. The warmer the air, the more moisture it can hold; the reverse is also true. When outdoor temperatures drop to -40 degrees C, a fairly high relative humidity still means that there is very little moisture in a given volume of air; that’s why our skin tends to flake away during cold winter days.

When temperatures are warm and relative humidity is low, plants lose a substantial amount of moisture through their leaves and stems. I have a hygrometer (a fancy name for a device that measures relative humidity) in my house that consistently showed R.H. levels of around 12-18% during the cold, dry days of January and February; the house temperature hovered at 21 degrees C. So even though my houseplants weren’t growing much thanks to the short winter days, they were still losing a fair bit of moisture thanks to the extremely low R.H.

Pebble Beachhead

One myth that continues to perpetuate with regard to humidity is that using a pebble tray will increase the humidity around houseplants. According to the theory, you get a saucer, lay pebbles in it, and put your potted plant on top of the pebbles. The pebble-filled saucer is then filled with water, but the pebbles are large enough to prevent direct contact between the drainage holes in the pot and the pool of water. Over time, the pool of water evaporates, theoretically increasing the humidity around the leaves, and therefore improving plant health.

It’s a great sounding theory, but like so many other great sounding theories, it doesn’t fit the facts. I’ve done several experiments with pebble trays and hygrometers, and I’ve discovered that a pebble tray has virtually no effect on the humidity of the air around the foliage at all. During those 18% R.H. days of January, one of my experimental pebble trays did indeed raise the relative humidity to 23% – but only one centimetre above the pebbles. At 10 cm above the tray, which is just about the height of the rim of a small pot sitting on a pebble tray, the R.H. was back to 18%. Even if the relative humidity could have been raised to 25% around the foliage, which is impossible to accomplish with a pebble tray, that would still be very dry air. The reason that many houseplants end up with brown leaves is not due to low relative humidity; rather, the leaf damage can be attributed to irregular watering, bad soil, and very poor root systems that simply aren’t capable of absorbing sufficient moisture.

Be Vigilant

To ensure that your houseplants will stay healthy, start by purchasing healthy plants with vigorous root systems, and keep those roots healthy by following a consistent watering regimen, one suited to the needs of that particular plant; this will prevent root damage. The price of growing great plants is vigilance, not one-shot cure-alls. Some may claim that pebble trays raise relative humidity; for me, all they raise is a sceptical eyebrow.

First Published 2/5/2004
Of Roots & Shoots

When we start plants from seeds in our home, we naturally assume that they are capable of striking the right balance between root growth and shoot growth. But this is not always the case. In fact, more often than not, this balance – referred to as “roots to shoots”– is a little out of whack. And to grow seedlings successfully, what’s happening below the soil is as important as what’s happening above.

Sizing Up Seedlings

The typical profile of a seedling that’s going to grow poorly once transplanted is a plant with stretched, soft, lanky leaves and shoots and “water roots,” also called hydroponic roots. Water roots develop when the growth medium contains plenty of water, but little oxygen, as if the plant were being grown hydroponically. Water roots lack the fine, fuzzy root hairs essential for healthy, vigorous seedling growth; they simply don’t absorb nutrients well, resulting in weak growth above the soil level.

The importance of the root to shoot ratio is most apparent when transplant time arrives. If shoot growth is disproportionately large relative to root growth, transplants will adapt poorly to the outdoor environment, often languishing for a short time before succumbing to the vagaries of our climate.

Shoring up Shoots & Roots

To strike the right balance between roots and shoots, follow these steps:

First, choose a high quality seedling mix, one that has plenty of air space, yet not so coarse that the seeds disappear into the mix. Seeds sown into heavy, dense, boggy, oxygen-depleted soil struggle to root. (We found this out firsthand years ago, when we pre-filled seedling flats with soilless mix, then stacked them, compressing the soil in all but the uppermost flat. The roots didn’t respond well at all to the compacted soil, and I find it amusing when people on gardening television shows firmly press seedling soil into flats. All this does is reduce soil volume and air space.)

Second, keep the soil warm and the air relatively cool. Warm seedling mix (20 degrees C or higher, depending on the seed species) is critical for good germination, but once the seedlings emerge, warm air temperatures cause excessively stretched, weak stems and leaves. And what’s worse, this extra stem growth comes at the expense of root development – the worst of both worlds. This puts a heavy strain on the seedling’s resources, since there are few roots to absorb nutrients. Seedlings in this state stand little chance of growing up to become vigorous transplants. For most plants, soil temperatures at emergence should be at least in the low 20s, while air temperatures should be around 16-18 degrees C. This can be achieved with a heating pad designed for growing seedlings and a spot in the house that is bright, but with relatively cool air, such as a well-lit spare room in the basement.

Finally, don’t apply excessive fertilizer to seedlings; it can burn off the fine root hairs. Many gardeners add phosphorus-rich fertilizers to their seedlings to increase root production – but, contrary to nearly all of the current literature, phosphorus actually has no net benefit when it comes to rooting at the young seedling stage. Phosphorus encourages lots of top growth rather than root development. In fact, in the greenhouse industry it’s very common to apply no phosphorus at all at the seedling stage. We’ve found that holding back the phosphorus, in combination with temperature management, helps keep the root to shoot ratio ideal. Only when the seedlings are transplanted into large containers should phosphorus be applied, and even then, judiciously.

Shooting for the Right Ratio

Don’t be too alarmed if you can’t attain a perfect root to shoot ratio. It’s really just a guideline, a useful tool for driving home the lesson that seedlings need a vigorous, well-branched root system, coupled with thick, stocky stems and leaves. If you choose the best seed, top quality seedling mixtures, keep the roots warm and the air a little cooler, you’ll be well on the way to seedling success.