Greenhouse Winter Clean-Out Technique

When your final plants have been pulled and it’s time to sweep-up the leaves and dirt, you look forward to the month or two when you won’t be carefully tending to plants in your greenhouse. You tuck a ‘pet plant’ in a corner or two to keep it in a comfortable dormancy and you shut it down.

Within a few days of having no work to do in the greenhouse you find yourself looking for things to do: repair a bench, replace some lights…etc. Few people, however, imagine spending the dormant time actively engaging in bio-control. But here you go: There are a couple things you should be doing.

1. Tidy: Remove all plant material (or as much as you can). Sweep, vacuum, use a leaf blower, pick it up by hand. Get it out.
2. Plant some bean plants in pots: Use “strike,” “provider” or any other thin-leaved bush bean. Do not put it in your greenhouse yet.
3. Wash: Gently hose off surfaces with water or add a little dish soap. Avoid using harsh chemistry. 
4. Buy Gaeolaelaps gillespie, a predatory mite and sprinkle it on the floor. Concentrate on areas where there are cracks in pavement, bare dirt, or where pipes and posts intersect the floor. You can also use Stratiolaelaps scimitus (outside of Canada) it does the same.
5. Put one or two bean plants into your greenhouse.
6. Sit back and enjoy the holidays. 
7. Grow more bean plants, but don’t set them out yet.
8. Check the bean plants you set out. Are there pests on them? Particularly spider mites? Destroy the plants and replace with new ones. 
9. When the time comes to reheat your greenhouse, or the late winter sun is starting to wake things up, begin a rotation of putting bean plants into your house, looking for pests, and discarding those plants when they become infested. 
10. Repeat as necessary until the bean plants stay clean.
11. Sow or bring in your new plants.
12. Buy Neoseiulus fallacis: A predatory mite and sprinkle it onto new plants and the existing bean plant.
13. You’re good to go!

On tidying: This is – to some extent – personal preference. If you deal with fungi, bacteria, or viruses, it’s best to clean the house out completely. You don’t want any chance of those coming back. However, in terms of pests, I’ve met successful growers who operate in opposite manners regarding tidiness. Some growers are perfectly clean – You don’t see a leaf on the ground, a weed, or even much algae. It’s remarkable, really. Those growers are counting on eradicating habitats that may support pest populations. The alternative you may find concerning, but I’ve also met other growers who wear gumboots in their houses as they wade through muddy low areas, and they pull weeds only when they are chest-height and easy to grab. There are no shortage of insects flying around or on every leaf, but there are good bugs and bad bugs. At the end of the day, yields are sometimes similar.

That being said, untidy houses are typically veggie houses, where the pest populations damage the leaves but never build up enough to kill them. The peppers, egg plants, tomatoes..etc never suffer any damage and so the fruit is fine. This would be a different story with cut greens, where a notch on a leaf is a missed sale or in cannabis where they are concerned about anything foreign. In ornamentals, I usually find a mix: Tidy greenhouses for propagation and early season annuals and more natural areas for perennials and some cut flowers. If you’re a home gardener, my guess is you have little plant material in the house, but a whole lot of potting soil!

On Bean Plants: The varieties “Strike” and “Provider” are proven to be preferential plants to spider mites. In this process you are using bean plants to “sponge-out” spider mites that would otherwise work their way into the soil, wait-out the winter and then return to your new precious plants. It is imperative that you continue to provide preferential plants all year. Not only do they attract pests that would otherwise find your precious plants, but you can use them as a “first-alert” scouting spot to know when pests have arrived. Need one for aphids? Take a sacrificial pepper plant and give it more fertilizer than the ones you want to keep. If your fertilization levels are appropriate, the aphids will prefer the sacrificial plant.

On Gaeolaelaps gillespiei or Stratiolaelaps scimitus: Both are similar. They work in different ways, but ultimately control the same pests. Spider mites need to work their way into the soil or other small areas to avoid freezing. It is there that your in-soil predatory mites will feed on them. Thrips pupate in the soil, and any that remained from last season will also be eaten. If there is leaf material on the ground with pests on it, both those predatory mites can leave the soil at night for surface feeding and will take care of some of those pests. The best part is they continue to work. They’ll also go deeper into the soil to avoid freezing, but as it warms, so will they. When larger pests like grubs and wire worms find themselves in the early life-stages in the soil, they will be controlled by the predatory mites. It’s a no-brainer. 

Bean plant placement: The options are endless here. Some pepper growers place the bean pots along the walk-way and the end of rows (sometimes hundreds of feet long). It’s so effective, spider mite will travel past those pepper plants to the bean. It also makes it easy to scout. During clean-out, the plants are most often placed in areas that had the greatest spider mite pressure the year before. If that’s not obvious, they are placed near intersecting pipes and posts, wherever the soil is accessible, and also in warmer areas (near heating pipes or south walls). For most of us, it doesn’t really matter. Put one right in the middle, or spread 4 out towards the corners. You can’t go wrong with equal spacing.

Checking the plants: You don’t have to put any in your house during the dormant period. But if you live in a place like I do, the house may never freeze, and a sunny day may push the interior temperature up to favourable levels. If that is the case, the spider mites will always be active and the bean plant is recommended. In a classic situation, growers don’t set them out until they have turned on the heat a week or two before their new plants arrive. Typically the first one(s) they set out will be covered in mites within 24 hours. In one case, a grower told me they were already covered in webbing. Those plants should be thrown out – maybe even burned. The next plants will be less covered and eventually you’ll have a clean plant. Ideally, it’s not totally clean, as you’re about to use them as food for another mite.

On Neoseiulus fallacis: The true, uncontaminated, healthy strain of fallacis is so effective its application rate is 2 mites per square meter (2 per 10 square feet). It’s ridiculously low, but they are fast, eat a lot, eat a variety of things and last as long as your plants do. Putting them on your “clean” bean plant means that any late-coming spider mite will be eaten, and you’ll sustain an effective population of fallacis. They’ll find other things to eat as well, so it is unlikely you’ll need to reapply, especially if you’ve applied them to the rest of your plants. The spider mites will easily be controlled, and you’ll also get predation of whitefly, other mites, thrips…etc. When your plants are set out into a garden they will bring fallacis with them and you get free inoculation of your yard or garden or for your customer to take home.

If you are going to keep “pet plants” that are outside all year, but brought in for the winter, know that they will be covered in pests. They will have nasty pests like brown scale, cold weather aphids, and spider mites. In a commercial house, it is advised not to bring in “pet plants,” but if you’re like me, some plants are like your kids and you’re going to care for them! If you will be overwintering these plants, do the tidying, and cleaning before you bring in those plants. Treat those plants with aggressive levels of cold tolerant bio-controls. Give them their own in-soil predatory mites (a couple of tablespoons of media), give them fallacis at a rate up to 5 times the normal rate and then give your house a small amount of Brown Lacewing (Micromus variegatus). They are cold tolerant and feed on most pests down to 4 degrees celsius and with winter light levels. They’ll take care of the pests that the mites don’t.

I mention not to use aggressive chemistry. If you sprayed (anything) last season, ensure you tidy and properly clean your house. Wash that chemistry away and wait a few weeks before applying the soil-mites. If you dealt with pests that are not controlled by biological methods, then you may need to apply a harsh chemical during clean-up. But, remember that no chemical will kill spider mite, so you’ll still have to sponge them out with bean plants and provide the predatory mites. However, if you take this route, use the chemicals early and then apply the mites as late as possible. The truth is no chemical is truly compatible. All will interfere with the predators more than the pests and you end up having to buy more bugs or continuing the chemical use the following year. It’s an expensive cycle. So ensure the plants you bring into your house don’t have the real nasty bugs!

It is best to keep a clean house, scout regularly, use preventative biocontrol and react when needed. Start the year with this clean-out plan and enjoy the success of sustainable, natural greenhouse growing.

Happy Growing!

If you want more information on using beneficial insects and mites, download my free ebook from Apple or in PDF version from our website.

Or read more at the following:

Micromus variegatus – The Brown Lacewing

Stratiolaelaps