Is there a best time?

The best time is morning. The next best time is when you remember! Mornings are best, in my view. If you’re watering deeply, as I suggest, the plant has a fresh reservoir of moisture to call upon for the day ahead. But, by nightfall, the ground and compost surface is dry meaning it’s more difficult for marauding molluscs to go pillaging through your plants. Why make it easier for them?!

But, mornings are hectic for most, so evening is a good second. Water will sit there until the morning as plants transpire far less at night, just a fraction of a hot day. Something I hear a lot is that at night ‘plants drink up excess water like a sponge!’ No. They don’t. They’re not transpiring. This is how water is drawn up from root to tip: 

“Water travels up through a plant, against gravity, from its roots to its leaves, through a network of Xylem vessels (are the pipework in plant stems that transport water and minerals from the roots to the rest of the plant). The pulling force that generates this movement is created by water evaporating from the leaves, in a process called transpiration” (RHS "How plants lose water" and "How plants absorb water" both a worthwhile read)

Signs of dehydration

Some plants wilt during the heat of the day. Hydrangeas and dahlias are a great example. Their withered leaves and drooping blooms convince you that they're gasping for water. But, by nightfall or the next morning, they’ve perked up again all on their own. This temporary 'wilting' is the plant's natural coping mechanism to deal with heat. They’re not dying, just taking a siesta! If they’re still droopy and wilted in the morning THEN they are in need of a drink.

Other plants, many shrubs and trees included, will show signs of water stress more subtlety. Evergreens can show browning or a bronze tone to their leaves. Deciduous shrubs and trees will look fine on top, but leaves begin to yellow and drop from the lowest branches. This pattern moves up the plant, until it's defoliated. An extreme response to extreme environmental stress. How do you get around all these facets of plant behaviours?

Well, it might sound obvious, but just take a look at your garden. Inspect your pots. Don’t just bowl into your garden after three weeks of a heatwave and wonder why everything is dead... Ok, that’s an extreme example, but getting to know the foibles of your plants is like getting to know a new group of friends. You do pick up on what plants need and when. It's the joyful thing about gardening. Every day is a school day!

TIP: If your pots have fully dried out and the compost has visibly separated from the inside of the pot, water will simply rush around the outside and flow out the bottom. The best way to rehydrate is to place the pot in a deep tray of water and leave it soaking overnight. By the morning, the compost should be fully moistened again.

Don't make a splash!

You’ve assessed your potted plants and you’re ready to water them! Now look at what is on the end of your hose... In most cases it's a 'spray gun'. Most have two options: jet or spray. Neither of which are actually great for watering plants. The jet is too hard and the spray too wide. I found a Gardena Lance that seems almost perfect. I’m not affiliated with Gardena. I’ve bought them. It's called the Comfort Lance. The lance gives me real control and the reach to water gently and precisely. It was designed for hanging baskets. The head has a narrow, fine, 'soft spray' and the other option is a soft constant 'bubble jet' stream - not a hard jet. There's also a trigger lock and an adjustable flow control positioned right beside your thumb, which is excellent!

Dry leaves are happy leaves

Spraying the leaves doesn’t help anything, apart from leaf loving fungal infections - think rust and blight! Even if it's an absolute scorcher, you're not giving the plant a 'refreshing shower', but slowing the plant's ability to transpire. Use a slow flow and drench the roots. Aim all the water around the base of the stems/trunk and the compost in the pot. If the surface is very dry, the water might appear to sit on top for a while. Not a problem! Moist compost and soil is invariably more absorbent than when it’s dry. So, pick 4-5 pots and water each pot briefly, so the surface is moist. Then go back and water them fully. You’ll see the water absorb much faster. If the water is just flushing out the bottom, use the above tip to soak the pot overnight.

Slow the flow

Don’t be in a rush. I know it can be boring. I know we have busy lives. But, if you just fill the pot up to the rim and hope it soaks through, you could be mistaken. Water always finds the path of least resistance, especially under pressure. So, if there are existing drainage channels through the rootball and surrounding compost, the water will just flow straight down these, again just pouring out the bottom. If you slow the rate, at least initially, the upper layer has a chance to absorb water, expand, and slow the rate of water passing through.

The weekly schedule I use, works on this basis. Slowing the flow. Yes, it takes some time. I have over 150 pots and planters and they are all drenched once a week (mostly form the water butts). But the plants thrive. If it's been a particularly wet week with heavy persistent rain, I’ll do the ‘lift test’ or ‘finger test’ first knowing some plants transpire at different rates.

Some find watering boring, but I enjoy it. I get to see the plants up close, spot any problems, watch the bees, or just enjoy their beauty and marvel.

How much water?

A great question! Fundamentally, it really depends on the size of the pot, how dry it is, how quickly the plant(s) transpire, etc. But here's a useful tip if you water with a hose:

1. Use the hose on full power (not a pressure washer) to fill a regular 9ltr (2gal) watering can.
2. Use a stopwatch to time how long it takes to fill. Or just count in your head like I do.
3. Now you have a 'volume standard'

In my garden it takes roughly 60 seconds to fill a 9 litre can. That's about 6-7 secs per litre. So, if I'm watering a pot with a similar size volume, I can count to 15 secs and know I've given that plant 'quarter of a watering can' or 2 litres. It doesn't have to be precise, but it helps to be consistent. The watering can is a great standard as it's practically universal in size and you can use it to compare all your pots.

Here are some examples, given that at a large proportion of the planter is filled with compost and roots. Remember, this is not 'gold standard' or even scientific. Just a standard I use to know how much I'm watering consistently. It's a very simple method and you can gauge whether you're giving enough water. Eventually you'll find a good standard that you can time or just count. Here are my regular 'counts':

• Our small 'Heritage' ceramic pots are about 10l, so similar to a watering can. I'll count to 15s giving approx. 2ltrs of water.
• My galvanised buckets are approx 14l, about 50% larger than a watering can. I'll count 30 secs, giving 5ltrs of water.
• Dolly tubs are roughly 100l. I count to 60 giving 9-10ltrs.
• The large cast iron boilers I have for my Acer palmatums are twice the size of a dolly, so I count to 120 for roughly 18-20l of water.

Be Proactive

Here in the UK, our weather can fluctuate wildly, even in the middle of summer. As I'm writing this, we've had a month of cool cloudy weather with occasional heavy showers and daytime temperatures hovering around 20℃ (70℉). But for one week in June and one week in July we've had mini-heatwaves with the thermometer soaring to 30℃ (86℉)!! So, a great tip for dealing with these extremes is to:

1. Maintain a regular watering schedule
2. Water your plants thoroughly before the forecasted heat builds. Really top them up!
3. Water in the early morning, before the heat of the day, so the water can soak in rather than evaporate.
4. Remember that your small pots may require daily watering OR bring them into the shade for the heatwave.
5. Once the compost dries out, it's very difficult to saturate. Your best option is keeping them well watered!

Irrigation

A quick word on irrigation systems. If you have lots of pots like me, but less time. Then irrigation could be a good option. It's a bit of a faff setting up. Timers don’t last forever. Rodents sometimes chew through the lines for the water. But, on the whole, they’re reliable and time efficient, if costly to setup. Irrigations can be connected to mains water (outside tap) or from a submersible pump in a waterbutt. The benefits of irrigation are twofold: the automation; the slow flow rate as they use 'drippers' with different/adjustable flow rates. 

Having dispensed with repeatedly failing timers, I use mine manually just to top-up moisture levels for our specimen Japanese Maples, Acer palmatum var. 'Dissectum'. Unless they're well-hydrated, they're prone to leaf scorch in blazing summer sunshine. Turning on the irrigation myself for 20-30mins, I can quickly see if there’s a leak! Once the irrigation is running, I can go about my business... remembering to set a timer on my iPhone first!

Hozelock and Gardena (my preference) are two of the biggest brands, but there are many other options and an infinite array of set-ups, water flow rates, parts and products. Irrigation is whole article (or two) on its own.

Drain or retain

Often overlooked, plates and chocks can either help retain water, or increase drainage. Plates sit under your pots and capture the overflow where it can be reabsorbed via capillary action. Chocks, are ceramic/terracota blocks that lift the pots off the surface, ensuring a clear gap beneath the drainage holes so the pot drains and doesn’t become waterlogged  - which happens a lot when smooth-bottomed pots are placed on smooth-surface paving. If it's a prolonged heatwave, I’ll break out the plates for the hostas, but otherwise most of mine sit on chocks to ensure proper drainage.