Articles about container gardening abound – from the owner’s perspective. Time for a change, time to look at containers from the plants’ perspective. The garden pot or container is an enclosed world that can be the life or death of a plant. Let’s take a closer look.
Table of Contents
The first photograph compares three different garden pots for the deck or patio plant, a 7.5 litre pot and two 30 litre pots. The large right hand pot is the standard 30 litre container. Notice how few drainage holes it has in comparison with the smaller pot, especially given the volume of the pots. Plants potted in this standard container could very easily drown because of being waterlogged. The remaining 30 litre pot shows the author’s handiwork with a 1 inch drill! The key to providing good drainage in any container is not so much the number of holes as their surface area.
Have you ever noticed a slight suction when you lift your container? Or a definite ring of water that mirrors the base area of your pot that remains long after watering? This is due to your container holes being too small for the watering received. When drainage holes are too small a miniscus or skin forms across the gap which can reduce water escaping. Add to this the containers’ weight and the minute gap between the container and its support surface, (deck, patio or bench) and the container is almost hermetically sealed! The miniscus can’t form across larger holes.
It’s common practice to put drainage material over the holes when filling pots, then covering the base drainage with whatever compost the plant requires. This is fine for pots 10 litres or under. But larger, deeper pots need to be filled differently with graded layers of compost.
If a 30 litre or larger pot is filled with the same compost throughout, the lower levels of the compost will compress over time with successive watering. With a large standard pot with standard-size holes, water will quickly drain from the top but collect in the lower half through the season. The gardener will ‘finger check’ the surface – it feels dry – and add more water! Thereby exacerbating the poor plants’ demise. The same gardener is later mystified as to why his large specimen plant succumbs during the winter.
The trick to filling larger pots and containers is to fill in layers containing increasing amounts of drainage towards the base. When filling, use about 70% drainage material in the lowest 3″ or 75mm then, in the next 6-12″ (150-300mm), depending upon the size of your pot, use about 40-50% drainage material/compost. Finally, in the top third, reduce the drainage to about 10%. This grading enables your roots to breathe more easily and even makes your pot lighter.
A good drainage material is ‘perlite’ or any similar inert drainage material that’s lightweight. Stones and gravel can be used. However, these are generally alkaline and most plants potted in larger containers prefer slightly acidic compost – azalea, bougainvillea and canna for example. Also, stones and gravel are heavier to pick up! You will notice the difference in 30, 60 or even 90 litre pots.
Pot plants prefer to dry out a bit in between waterings, just so the compost is ever-so-slightly moist but not ‘arid’. Even if it’s just a couple of days, it gives the roots a chance to breathe. When watering in summer take care to be sure it gets there! Water, pause, water more, pause again, water again is more effective than the occasional deluge that simply runs on to the deck and does no good at all.
Try to avoid the ‘glut-famine’ regime often recommended on feed packets which can be stressful to plants. Far better to reduce the amount of feed offered in diluted form to your plant but add it more frequently, say every week.
No matter how well a pot plant gets fed and watered, the soil will stagnate and some roots will die off. It’s always a good idea to periodically check your potted root systems , remove unhealthy looking roots and about every three years renew the soil.
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