Grow Tent Mistakes: 10 Things Most Growers Get Wrong

The rumour mill never really stops. Some of it's honest misunderstanding, passed on in good faith. Some of it comes from companies stretching the truth to shift a half-baked product.

Either way, we hear the same ten things over and over — and most of them will cost you yield.

Here's what's actually true.

Should your nutrient solution feel warm?

No. It should feel cold — somewhere around 18–21°C.

That's warm enough for healthy growth, but cool enough to keep oxygen levels up. And oxygen is what keeps roots healthy and holds off some genuinely nasty root diseases.

Let your solution get too warm and oxygen levels drop. Algae loves a warm tank too, and once that takes hold your oxygen drops further still.

Is too cold better than too warm?

Also no — and this one catches people out every winter.

A cold room or a cold tank will hurt your yield just as surely as an overheated one. Aim for:

• Nutrient solution: 18–21°C
• Room temperature: 24–28°C

Don't be shy about using a nutrient heater or a grow room heater to get there. There's a full guide in Cold War Heroes – How to Use Nutrient Heaters, and 9 Ways to Fight the Freeze walks through a proper winter health check.

Should you stick to one fixed nutrient strength?

Nope. There isn't a universal number, and here's why:

• Tap water CF varies enormously across the UK
• Every plant has different needs
• Your environment changes uptake — more light means more food

That's why most manufacturers give you a range rather than a figure. As a rough starting point, use a quarter to half strength for young plants and half to three-quarters for larger ones.

Full strength for most manufacturers works out around 1200ppm, or CF17 with water at CF 0. That suits most species, but tomatoes often want a bit more and lettuce wants a lot less. Test, adjust in small steps, and watch what the plant does.

Worth reading alongside A Beginner's Guide to CF Control.

Are rusty spots and colour loss always a deficiency?

Probably not — and this is the one that costs people the most money in bottles they didn't need.

Unless you're seriously over or underfeeding, colour loss and rusty spotting usually come from something environmental. Especially when you see them together. The usual culprits:

• Lights hung too low
• Nutrient solution too warm
• Poor air exchange
• Wind burn from a fan pointed straight at the canopy
• Nights getting too cold
• A grow room that's simply too dry

Check those before you reach for a bottle. How to Stop Leaf Damage goes through it properly.

Is there one reason plants stretch?

There's rarely a single reason, which is what makes stretching so frustrating to diagnose.

It might be too little light, so they're reaching for it. It might be too much light or too much heat, which happens when your lamp sits too low. Or it might simply be genetics.

Start with hanging height, because that's the one you can change today — see Lesson In Lamps: Hanging Heights.

Should you adjust pH every day in a recirculating system?

Never do this. Genuinely, never.

Your pH will drift upwards day to day, and that's unavoidable — especially with rockwool slabs and hard water. But if you drag it back down every single day you'll end up with nutrient lockout from too much phosphoric acid.

Let it drift for a day or two instead. It's by far the lesser of the two evils. Constant adjustment does more damage than the drift ever will.

More on this in A Beginner's Guide to pH Management.

Do grow tents keep the smell in?

The tent doesn't. Your extraction does.

A grow tent isn't sealed and isn't meant to be. What controls odour is a properly sized carbon filter, pulling air through it, with the tent under slight negative pressure so nothing escapes through the seams.

Here's the quick check: look at your tent walls while the fan is running. Pulled gently inwards is what you want. Bowing outwards means air is being pushed out through every gap, and no filter will help you.

If smell is escaping, work through it in this order — pressure, then fan size, then filter age, then your joins and clips. STOP Odours Escaping covers the whole subject.

Do you need an intake fan in a grow tent?

You can run either. It depends on your fan, your filter and your ducting — so it's worth understanding the difference rather than following a rule.

Passive intake means a mesh vent or an open port, and your extraction fan does all the work. Air gets pulled in because the fan is pulling air out. It's simpler, cheaper and quieter, with one less thing to fail or to wire in.

It works well when your extraction fan has enough grunt to cope, and when your passive vent is big enough — a good rule is roughly three to four times the area of your extraction ducting. Undersize that vent and you'll choke the fan.

Active intake means a second fan pushing air in deliberately. That gives you control over how much comes in and where from, and it takes the strain off your extraction fan.

It earns its place when your extraction is working hard already — a carbon filter, a long duct run and a few bends all add resistance, and plenty of fans can't pull enough through the lot. It's also the answer for larger rooms, and for anyone who wants to balance pressure precisely rather than leave it to chance.

Which way to go: watch your tent walls with everything running. Pulled gently inwards is right. If they're sucked hard inwards and the fan is straining, your passive vent is too small or your fan is undersized — that's the point to consider going active.

One thing that applies either way: mind what you're drawing in. Cold outdoor air brings temperature swings, humidity and bugs with it, so think about your intake source rather than just the method. Extract outdoors by all means.

Why Every Indoor Grow Room Needs an Active Intake Fan makes the case for going active, and Grow Room Ventilation covers sizing and installing the whole system.

Where should you put your thermometer and humidity sensor?

At canopy height, in the middle of your tent — not on the floor and not clipped to a wall.

This sounds fussy. It isn't. Floor readings can be several degrees out from where your plants actually are, and a sensor on the tent wall reads the wall, not the room. If you're making decisions off a number that's wrong by three degrees, everything downstream is wrong too.

Keep it out of direct light and out of the airflow from a circulation fan, and move it up as your plants grow.

Once you're confident your readings are honest, VPD in a Nutshell shows you what to do with them.

Can you put a heater in a grow tent — and are tents insulated?

Yes to the heater, and no to the insulation.

Tent canvas insulates almost nothing. It blocks light and it holds your extraction under negative pressure, but it won't hold heat in — so don't expect the tent to do a job it was never built for.

Before you buy a heater, though, plug the cheap leaks:

• Get pots off cold floors. Concrete pulls heat out of the room and out of your root zone.
• Sort your intake source. You can't draw in air warmer than the space it comes from. An unheated garage sets your floor temperature.
• Check your ducting. Long uninsulated runs through cold spaces bleed heat. Acoustic ducting holds it better than combi.
• Seal the gaps. The same leaks that let smell out let heat out.

Then a tube heater on a thermostat, sized to your space, has far less work to do — and costs you far less to run.

Get it right first time

None of these are complicated. They're just the ten things we spend most of our time correcting across the counter.

If you're not sure whether your setup is working as it should, come and see one running in any GroWell store. Bring photos, bring your numbers, and we'll go through it with you.