Starting from Seed – the relationships between light, heat and bugs.

If you’re like me, the first sunny day after New Year’s is the start of the growing season – at least in my head. I’ll wander the garden looking for some gardening task, and finding none, I’ll go inside and start thumbing through seed catalogues. Then from week 1 to mid summer I’ll be seeding trays indoor and outdoor and direct sowing into my garden. It’s a busy time of year… and I love it.

The early start works “ok” for me. Our average last frost date on the coast doesn’t mean much. It could be 14ºC in January, and 5ºC in May. We could have our last freeze in January or in April. So, for me, it’s a gamble. My seed schedule is based on sowing directions and the average last frost date, but I don’t feel the need to wait for that date to set out the plants. Instead, I’ll put them out months before (the gamble) or at the Historic Last Frost Date, when I don’t feel like the risk is worth it.

It’s the early seeding that I am writing about today. January is a long time from when my plants will go outside. What that means for the plants is low-light and an increased chance of both pests, and promoting well-established pests. So, let’s dig-into light, heat and the bugs that will be the major players in our seeding success.

Before we start, let’s go to the very beginning: seeding starts with a growing media. I’ll save the discussion about which media is best for another time, but in the meantime, know that new, seeding-specific growing media is a good place to start. Do you need it? No, but it’ll make your life easier. Wet the media, and stuff it gently into the container you intend on growing in. The best soil for roots is thoroughly damp, but not wet. Make a hole or divot based on seed-depth recommendation in each cell or pot. Cover with soil as recommended and cover the seed tray or pot with a clear plastic that is either vented or will be removed regularly to reduce moisture build-up. Add Stratiolaelaps scimitus (predatory mites) to the soil to prevent fungus gnats from establishing. And now, read-on.


The sun is irreplaceable; It is the least expensive, the brightest, and offers all the wavelengths that your plants have evolved with. Is it enough in January? Or is it enough here on the West Coast where I can expect as many cloudy days as not? No. So we must supplement our seedlings. If you do not supplement with light you will get leggy, weak and susceptible seedlings.

The light brand does not matter. The light quality and quantity does. You will notice that light is measured by lumens or lux. For both, the greater the number, the greater the light. However, to better understand light we have to break that down a little:

Lumens are the total output of light from a source. So a high lumen light is going to look bright.

Lux is the total light intensity over one square meter of surface. Lux is important for growing plants and in most cases will be controlled by how far away your grow lights are from the top surface of the plants.

Wavelengths have been a popular discussion since LED’s allowed us to more efficiently light our surroundings. The human eye sees wavelengths between 380 nanometers and 780 nanometers. And because of that, most lights are measured by how bright the light is, according to the human eye. So, the problem is, we can have very intense lights outside of our range of vision, and while it will look dark to us, it is likely very bright for the plants. The perfect example of this is LED’s made of both ultra violet (or far blue) and infra red (far red), but omitting the wavelengths in between. In rooms lit like this (I’m thinking indoor grow-ops), it seems dark, and everything is pink/purple, and the stoned staff are all wearing sunglasses to protect their eyes. There are plenty of reasons to grow plants like this, but don’t jump into that without reading further – I promise it’ll be worth your time. The Sun emits all wavelengths. And yes, if plants look green to us then they are technically reflecting(and not absorbing) the green part of the spectrum, but it’s a little more complicated than that. (Think about looking up through a green canopy of leaves – obviously some green light filters right through.) What I will tell you now, and expand on a little more is that plants need it all. Supplementing with specific wavelengths is sometimes warranted, but, always start with pure sunlight and consider supplementing with pure white light first.

In a massive operation where your lighting is a major capital expense and a noticeable operating expenditure, then saving energy by not using it for the full, sometimes wasted, spectrum might make sense, but I’m assuming (with the low cost of LEDs these days) that you can afford a decent grow light.

If you don’t want to use LED’s, fluorescent lights offer a suitable wavelength but are weak, and old-school halogen or HID lights are very bright but might require you to rewire your house and pay a lot on your electrical bill.

Now, don’t forget day-length. If you leave the lights on for 24 hours you will continually activate the chlorophyll and other photo-reactive chemicals. But you are doing a major disservice to your plants. Plants (and bugs) need a night period. The best way to explain this is the (now well known) study that found increased Carbon Dioxide in the Amazon at night. What they found was at night the jungle reversed some of its Oxygen production and instead released Carbon Dioxide. In short, a dark period is essential for many plant functions. A study looking at Cannabis and other plants for comparison found that as soon as it was dark some phyto-reactions immediately stopped and would immediately begin again when the lights came on. However, other metabolic activities would slowly stop over the course of several hours. If the lights came on, it started again. So the lights HAD to be off for specific times in order for all the natural processes to occur. So, to put it simply: Don’t mess with this. Make you plants think it is Spring: Over 12 hours of light, and the rest total darkness. (Dusk and Dawn are also important, but we won’t get into that today.)

So, to recap: The lights you want are LED, white, and adjustable to lower and raise them above your seedlings. Aim for an intense 12-14 hours of light and then dark for the rest of night.


If your seeds are cold and wet they may rot or not come up at all. That being said, you always want to start them on the low-end of what you’ll have success with. Yes, warmth will speed things up, but trust me, patience is a virtue, even here. I start seeds three ways: In a cold-frame greenhouse that can get as low as freezing and as high as 15ºC during the day (maybe even 20ºC). In this cold house I plant early spring, cold loving plants. (Spinach, arugula, mustards, lettuces…etc). Second, I grow in my actual house by a window. This (if far enough from the window) should stay around 20ºC. Here I need to start any plant that needs a head start because of my climate: tomatoes, peppers, onions…etc. Lastly, I direct sow. This is usually just a battle with slugs so I sow aggressively and trap aggressively as well.

Ideally I’d grow each plant at a very specific, optimal temperature for each, but that’s just not possible. So those three general placements have done me well. However…and this is a big HOWEVER….one year was so dark and cool for so long that biannual plants like brassicas and carrots that I had started indoors, but moved to the cold frame thought they had gone through the winter and bolted early. Even though they never froze, they obviously had been tricked by the reduction in warmth and light when I had moved them out. So consider this:

Grow your plants like it is early in the growing season and only going to get brighter and warmer. This is natural for them, and it will avoid you making the mistake I did.

The Relationship between Light and Warmth

Now for the real important stuff. If your seedlings come up leggy there are two things you can correct: more light or less warmth. Leggy plants are experiencing “shade avoidance.” This means they believe it is late in the growing season (because of the warmth) and that they are behind in growing, and being shaded by larger plants (because of the low light levels). So a competitive drive kicks in which produces a whole lot of stem very quickly in order to get up to that light source. Most people will experience leggy plants and simply increase the light. An obvious sign of that need is your seedlings bending towards the window (sun). Increasing the light will straighten the plants and produce bigger and broader leaves instead of longer and narrow stems. However, what can also be done is to reduce the temperature a little (and not all at once). If the plant detects lower light but lower temperatures it will know it’s early in the growing season and slow down, making better roots in preparation for better growing conditions. But don’t go extreme. Too cold and too dark simply stops the growth and opens the doors for pathogens. As I said earlier, find the coolest your plant is happy to grow at. And that’s because…..

The relationship with Bugs

Bugs love a weak, fast growing plant. A warm plant without enough sun to help it out is going to be susceptible to pathogens and invertebrate pests. The plant will be soft and without its natural defences built-up. A more patiently grown plant will have better roots, tougher leaves and stems and a developed protection system against pests (usually chemical responses). But this DOES mean your plants will not be as big. So, right now, you need to stop measuring the success of your seedlings by the height of their growth. Instead, measure your seedling success by the health of the plant (root development, compact growth with large leaves, and resistance to pests). Furthermore, as a general rule, the warmer it is the faster insects and mites can be. And, since you’re growing protected plants, pests will smell them and try to find them, predators will not. So you can end up with a pest situation quickly and it can intensify quickly. While cooler temperatures do not promote the native predators, it does reduce the amount of plant volatiles inviting the pests, and inhibits pests’ ability to spread quickly.

Let me be clear that this is a major generalization. There are famous pests like the “Foxglove aphid” that is very effective at temperatures below those that are suitable for most commercially available predators. Therefore, foxglove aphid can get going in a cool greenhouse easily, and the best course of action might be to warm it up so that predators can go to work.

Now, back to the lighting. There are numerous recent studies about wavelengths and plant growth. But even people who have read them generalize their own conclusions and can make mistakes. For example, a study using one species of plant might find that they grow best (based on height or mass) with wavelengths that favour the blue wavelength. It was studies like this that made people all go out and buy “purple” grow lights. But other studies using other plants (and one in particular that looked at plants that flower in Spring, summer or autumn) found that the wavelength combinations caused different growing behaviours in each plant. So, circling back to early in this article, just go with white light. The limiting factor might be a specific wavelength, but unless you know exactly what that is, you’re not doing any harm…..but you might be if you, instead, omit the other wavelengths. For example, other studies have shown that while plants grow “fastest” when light is specifically blue and red, those plants were more susceptible to some pathogens. I’ll reiterate that there are tons of studies on plants and wavelength, and it’s all relatively new because only LED’s have made this research doable.

The same is not true for studies on wavelengths and bugs. Only one study comes to mind, and that is by Rose Labbe at Agriculture and Agri-Canada. She set out to look at whether or not the exclusion of wavelengths might induce different behaviours by some predators. However, this is just the tip of the iceberg. Each pest and predator potentially reacts differently. Some wavelengths might induce diapause (hibernation), or stop oviposition (egg laying) or might just prevent them from seeing where they are going. What we need to know is: does it affect pests, and if so, how. Then we need to know if it affects the predators/parasitoids. I believe some of this work is being done as I type. But the pragmatic lesson to be learnt here is that the natural-state is likely the best balance between plant, pest and predator, so stick with sunlight and white light until we know for sure.

Lastly, I’ll touch on day-length again. Remember, I suggest treating your plants like it is Spring. And this is for the bugs too. And here’s an example: Aphidoletes aphidimyza is a natural enemy of most aphid species. It does not show up naturally outdoors until the end of April or May in most of the Northern latitudes. Because it is commercially available, it can be used all year around, but there’s a catch:

From May to September, Aphidoletes can be released and each of its larvae that eat enough aphids and pupate will emerge again as an adult two weeks later. That means you can by a little, and let them naturally multiply.

From October to mid April Aphidoletes will still fly around and lay eggs, and their larvae will still eat aphids, but sensing day lengths of less than 14 hours, they will (actually eat more aphids and then) pupate as if they were to try to survive the winter as pupae. They will not emerge again until soil temperatures reach late-spring temperature. Therefore, when using them in the dark months they must be used regularly, or supplemental light must be provided to make a day length of greater than 14 hours.

Similarly, some ladybug species feed and reproduce in a very tight time frame. One species from Europe begins a lengthy diapause in June. So its trigger for diapause is extended day length. (I have simplified that for this discussion – but most ladybug diapause lengths are fixed, so they diapause for exactly the same number of days, but when they start can be artificially manipulated, so their internal clock is part of the equation.


Grow your Winter and early Spring seedlings as if it is only slightly later in the year. Provide them with a spring-like day length (14 hours perhaps) and give them intense light as a supplement to daylight, and reduce the temperature to the low-end of their ideal temperatures. And know that the cooler temperatures and more patient growth patterns will help protect against pests, make them more cold-hardy at set out and can increase root development. Never forget to protect your seedlings with good quality growing media, adequate watering (and never over watering) and beneficial mites in the soil to protect your roots.

Should you get pests, please consider reading some of the following and check this out or share the information in video format here.

How to Control Aphids

Free Good Bugs through Conservational BioControl.

Everything you need to know about Mildew

Everything you need to know about Aphids

How to Control Thrips

How to Control Spider Mites

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