Posts Tagged ‘window box’

Building a Hydroponic Raft in a Window Box

hydroponic window box

Let me start by explaining my interest in this: I live in a flat, I have no garden and the waste land I tried to reclaim round the back of the garages the building owners cover it in pesticides. That leaves me one alternative, getting an allotment! Sadly the queues for that are too long and it can take years now to get one, the other issue is that I am incredibly lazy and don’t have much time to take care of an allotment, or any sizable garden for that matter.

So I have no space, I’m lazy and don’t have the time. This leaves me with growing things on my window sill, but this causes an additional few problems, there is reduced space and generally you need deep pots to ensure ample space for good root growth. I also seem unable to get the watering right, either too much or just not bothering at all. If my plants have survived this long then they’ll either get green fly, or more likely lots of tiny flies living in the soil. So growing on my window sill in soil also has problems…

This is probably enough to stop most people, but I figured there had to be a better way that would take little effort and minimal space. Enter the wonderful world of Hydroponics. Hydroponics is viewed as an expensive option to grow your own food since it typically has a high costs to start and requires specialist fertiliser, so growing a lettuce hydroponically will cost far more than if you bought it down the shops – but that’s the beauty in economies of scale I guess. For me the goal is to grow as much as I can in as little space as possible.

Introducing the Hydroponic Raft

The simplest hydroponic construction is the raft, basically you have a tub of water and your plants float on top of it with the roots dangling down into the water. You add nutrients into the water and hey presto, your plants grow and you don’t need to worry about water levels that often since everything is floating.

Which Plants to Grow

The raft method only suits water loving plants, this leaves you with things like salad greens such as lettuce, chard and spinach. I haven’t tried much else in this but I have to say Spinach really loves it. Get your plants started either in rockwool or jiffy pellets to keep things easy.


hydroponic raft parts

Hydroponic Raft

  • Flower box/ trough, needs to be water tight, can get them from the hardware store – make sure it’s sturdy and not likely to bend/ warp.
  • Polystyrene, about 5cm thick you can get this from hardware stores for loft insulation.
  • 3inch diameter net pots (order on ebay etc…) or use some flower pots as long as they have holes in the bottom of them. They need to be just a bit deeper than the polystyrene
  • Growing medium – clay pebbles Hydroton, reusable and cheap but you’ll need to buy on line. Remember to wash and rinse the clay pebbles to get rid of dust.

Hydroponic Nutrients

general hydroponic nutrients

I spent a long time researching nutrients and every one of them come in multi-bottle formulas requiring you to mix them, finding the amounts to mix is near impossible, even from the manufacturers leaving you to trial and error and peoples opinions in forums. So I went with the only manufacturer I could find that had sensible information and a mixing calculator – General Hydroponics. I had to order this online and it comes in 3 bottles, this by far is what costs the most.

Water Quality

ph test

You’ll also need to check the pH of the water and a way to alter the pH. I recommend a small bottle of pH indicator and some ph Up and ph Down from Growth Technology. Just be sure that if you buy it on ebay that the muppet who sells you it ensures the bottles aren’t leaking as these are concentrated acids & alkalis.


  • Aquarium air pump
  • Air hose & air/bubble curtain

This helps to oxygenate the water for better root growth and to inhibit fungus/ mould growth.

You can also add in lighting but I figured start by building the system first.


Drill & 3 inch hole saw (68cm).

Syringe to extract and measure pH adjustments and administer nutrients – you need one up to 100ml.

Something to cut the polystyrene, I know that’s quite vague but it’s so easy to cut that you can do it with anything you can find – you’re only limited by your own patience. Use a saw, a knife, you can build a hot wire cutter with a bit of wood, a car battery and some wire or you can use an electric carving knife and make a huge mess on the floor like I did. Be warned, you will make a mess, do not let your girlfriend witness the mess. Be sure to also have a vacuum cleaner on standby, this stuff gets everywhere.

cutting polystyrene mess

Building the Hydroponic Raft

Measure the flowerbox and divide this into a few smaller sections, the idea is to create a group of floats so that you can lift a section out of the water for maintainance. Draw out your sections on the polystyrene, there’s no need to be too accurate as you can trim the sections down to fit.

Cut the sections down to size – most flower boxes are tapered, so you’ll need to angle the sides in a bit to fit better.

drilling polystyrene

Once done, mark out the holes for the pots – you want to cut the holes just a bit smaller than your pots so it’s a snug fit. I spaced these pots about 4 inches apart from their centres, giving the maximum space for the plants to grow (depending on what you’re growing)

planted rafts

Insert the flower pots or net pots if you’re fancy like me. Fill them with the clay pebbles and add in your seedlings. For this batch I grew my seedlings in jiffy pellets then just added that in to the pot in amongst the pebbles.

Add in the water up to 3 cm from the top of the container, measuring how much water you’re adding. Use the General Hydroponics nutrient calculator and mix in the required nutrients based up on the water volume and desired growth rate, leave it to stand for a couple of hours and check the pH and alter it accordingly – you only need a very, very small amount of acid or alkaline to alter the water pH and you’re aiming for a pH of about 6.5 which in a colour indicator looks like urine (yellow). As an example, for 20 litres about 3-4ml of acid will take it from a pH of 8 to 6.5.

hydroponic window box

If your adding in the air pump, you just need to lay the air curtain at the base of the box, they normally don’t float so it should be fine.

Lessons Learned

The raft was very easy to build, but monitoring the water was a hassle since you need to remove a raft to check it – I wasn’t using a separate reservoir¬† as space is limited.

hydroponic raft spinach-4 weeks on

The plants grew rapidly, but I followed the nutrient calculator to the letter and ended up adding too high a nutrient quality which meant I got my spinach flowering and some stunted growth in other plants – the only way to fix this is change the water and start again. So I maintained a fairly weak nutrient mix.

I also found that the volume of water, 23 litres to be exact, was more than enough to buckle and warp my cheap plastic flower trough permanently after a couple of months, so I would advise something much sturdier.

warped flowerbox

The clay pebbles I used actually need to be rinsed and washed to get rid of dirt as this causes problems with the roots and also your air pump/ air curtain if you’re using one.